Microphones were concealed inside the light fixtures of a California resort hotel, their wires running through hollow walls, down an elevator shaft, and into a small ground-floor room where two soldiers wearing headsets transcribed what Japanese prisoners of war were saying to one another upstairs. According to the Army’s report on the facility, every building used by the prisoners was equipped with listening devices—not just interrogation rooms, but hallways, sleeping quarters, and bathrooms. There were even pickup devices hidden in trees on the lawn, in the part of the grounds where men went to walk alone, believing no one was watching. In 1944 alone, roughly 921 Japanese and 645 German prisoners passed through the two sections behind the inner fence.
The facility never held more than 51 people at once, usually two per room. That was not a housing decision; it was an intelligence-gathering decision, and the entire building had been rebuilt for that purpose. The hotel sat at Byron Hot Springs, in flat agricultural country east of Contra Costa County, about 20 miles west of Stockton. On paper, it was a temporary holding center.
Mail was sent to Post Office Box 651 in Tracy, California, and everyone who served there called the place “Camp Tracy. ”
What created it was a War Department memorandum dated May 12, 1942, and what that memorandum asked for is worth pausing over, because the ambition in it was very small, while its results were anything but. Before all of that, the springs had been a spa. Fifty-seven mineral springs rose from the ground, sulfurous and warm, and a hotel had stood over them since 1865.
Two fires destroyed earlier buildings, one in 1901 and another in 1912, and the owners rebuilt both times. A brochure published in the 1870s promoted the waters as a natural health resort for all of California and the Pacific Coast. Southern Pacific trains stopped at the station seven times a day. In good years, the guest register included film stars from Los Angeles, ballplayers, and some of San Francisco’s wealthy.
Then the Depression killed the business. The doors closed in 1938, and by the time the Army inspected the property, it had been vacant for four years. The memorandum called for an interrogation center on the West Coast, and Byron was chosen. Ten thousand dollars were allocated to acquire and equip the site.
Sacramento District engineers handled the renovations and new construction. There were only two such centers in the entire country: Fort Hunt in Virginia for Germans, and this one for the Pacific. The War Department listed both as temporary holding stations for prisoners captured by either service. In March 1943, the War Department leased 209 acres from an investment company and six more acres from a man named Tillio Morchio, plus a small easement from the power company.
The engineers renovated the hotel and 11 cabins, built a pumping station and a two-bay firehouse, added a garage, and laid several miles of underground drainage pipe. Their files never settled on a name for the place; some paperwork called it the Byron Hot Springs Holding Center, and other papers called it the Byron Hot Springs Interrogation Center. They also wired the entire place. Behind the inner fence, the ground was divided into two sections, one for Japanese prisoners and one for Germans.
Each included walls covered with maps built from what interrogation had produced. Each had rooms for code work and a room where someone listened. Outside that fence were the usual fittings of an American base: barracks, mess halls, a ball field, and Colonel Rhodes F. Arnold.
The ground inside answered to the head of military intelligence and almost no one else. The camouflage mattered as much as the wiring. Under the Geneva Convention of 1929, a prisoner’s government had the right to know where he was, and neutral Swiss inspectors had the right to visit him. A facility that held men quietly for weeks while their private conversations were transcribed would not have survived that condition.
So the Army listed both centers in its records as processing stations, places where a prisoner stopped briefly on his way to a permanent camp, with his paperwork completed once he left. Swiss representatives came to Byron from time to time. They were shown a rest stop. What happened inside would not have matched the expectations of a 1943 visitor either.
Japanese cooks came to prepare Japanese food. Prisoners used the mineral and mud baths, the same waters that had once drawn filmmakers, and some went to nearby towns for civic events. About 30 Nisei soldiers—Japanese Americans who spoke the language fluently—worked in the Japanese section alongside white officers, often speaking the language perfectly while pretending in front of the prisoners that they did not understand it. No one at Byron had to hit anyone.
The Americans running the place, taught by the British first, had concluded that comfort produced more useful information than pressure, and the entire building was designed around that conviction. The organization behind it, a branch of the Military Intelligence Service, had modeled its structure on the British office that handled enemy prisoners, down to making surveillance a standalone function rather than an afterthought attached to interrogation rooms. Someone was always on the listening post. A session record was kept whether the man upstairs said anything or not.
Upstairs, between every two rooms, there was a vent placed at a height that allowed a man standing on a toilet to look into the next room. Researchers who examined the building more than 60 years later believed that was deliberate. A prisoner who found a way to talk to the man on the other side of the wall thought he had outsmarted his guards. And a man who believed that spoke without watching his words.
Everything he said traveled through the vent to the transcription post. The same logic placed a second prisoner in his room from the start. Sometimes that second prisoner was an SS soldier wearing a borrowed uniform and a cover story prepared in advance. The Army had bought a health hotel, rebuilt it around a listening system, and what it wanted was talk.
Not every man who surrendered ended up there. Camp Byron was established for intelligence at the national level, which practically meant a prisoner who knew something technical, industrial, scientific, or political that outweighed the importance of the next island. Field investigators identified such men where they fell, while everyone else went to a regular camp. The route a selected man took was long.
He answered questions where he was captured, then again at camps in Honolulu, then faced evaluation at Angel Island in San Francisco Bay before the Army moved him inland to Byron, and afterward to Camp McCoy in Wisconsin. The Joint Intelligence Center, an Army-Navy body, ran this chain, with the captured personnel and materiel branch of the Military Intelligence Service handling the Army’s share. Over the life of the operation, nearly 3,000 men passed through its gate. Some of what was extracted from them was much better than mere talk.
A Japanese naval officer interrogated at Byron gave details on the design and arrangement of Japanese warships, their armament, and their actual capabilities. American intelligence kept working from his answers until the end of the war. A sailor taken from a damaged submarine near the Aleutian Islands described those that passed without leaving a trace on the surface. A soldier guided his interrogators inside a Mitsubishi plant in Nagoya, identifying the buildings that housed Raiden fighter engine shops and those reserved for final assembly.
American bombers used that information in the final months of the war. There is a reason so much of this worked, and it had nothing to do with mud baths. Japan did not recognize that its soldiers had become prisoners. A man who surrendered was struck from the records of his homeland, and his family was told he had died, so he believed he had lost his country along with his name.
No Japanese soldier had ever been trained in how to behave in captivity, because the army that trained him refused to acknowledge that capture could happen at all. He arrived at Byron with no prepared plan, no cover story, and no expectation of ever returning. The Americans interrogating him understood all of that and built their approach on it. Strauss, who spent years studying the interrogation files, summarized the result plainly: most Japanese prisoners answered without any preparation of any kind.
Against all of this stand numbers from the sister facility in Virginia. One man who worked the listening post at Fort Hunt, listening to German prisoners day after day, estimated that useful speech accounted for about a fifth of what he heard. The rest, he said, was about girlfriends, sergeants, and people back home. There is no particular reason to believe the tally at Byron was different.
Ulrich Strauss, who reviewed the interrogation files at the National Archives, concluded that the camp’s true output value is hard to determine. Thousands of men passed through for a few weeks each on their way somewhere else, and most of them said nothing of value. The machinery worked. What it picked up on any given night is another matter, and the surviving documents do not settle it.
The equipment was not proof against a careful man, either. At Fort Hunt, a German prisoner noticed a strange echo in his cell and said aloud that he thought someone was recording them. He and his cellmate dismantled the light fittings until they found the microphone. The duty listener had to sit and listen to them do it.
Nothing in the Byron files describes the same thing happening there, which does not necessarily mean it never did. There is something else the files carry without explanation. The son of a dentist stationed at Byron recalled that his father mentioned gas masks, stained inside with rotten onion, that were placed on prisoners to make them talk. Alexander Corbin, the military intelligence officer who wrote the camp’s history, had heard the term used vaguely, searched for it, but found no details or confirmation in the written interrogation files.
If onion masks were ever used at Byron, the official records do not admit it. Among the later arrivals were likely a U-boat crew brought across the country in 1944 because they had killed one of their own. The crew was under severe pressure and gave up nothing. The final months were the most productive, and that is the uncomfortable side of this entire story.
Very few Japanese fell into American hands alive in the first two years. Camp Byron spent its early period handling the overflow of German prisoners from Virginia. By the time Japanese prisoners were numerous enough to justify their own section, the war was nearly over. Medical officers interrogated in 1945 gave information about Japanese biological warfare research.
Men captured at Iwo Jima identified munitions factories and codenames tied to specific military units. Others confirmed, without pressure, that the home islands would be defended to the end if the Americans landed. Then time ran out for the building itself. The Army declared Camp Tracy surplus, partially canceled the lease on August 1, 1945, and ordered the site closed by September 1.
Equipment worth keeping was moved to other facilities. The wires stayed in the walls because removing them was not worth the labor. The lease termination procedures took longer than the camp’s operation itself. The smaller plot returned to its owner in January 1946.
The main 209 acres remained on the books until the last day of May 1947, with the government paying to restore the land to its original condition, and the power company easement was not relinquished until February 1948, two and a half years after the last prisoner left. Sixty years later, a Japanese television crew found the other half of the story. Researchers spent months at the National Archives in Washington reviewing Byron’s interrogation files. They copied the reports, identified the men named in them, then went home and tracked down many of them in Japan, by then in their eighties.
Capture carried a stigma at home, and most had lived with it in silence since. Nearly none of them had discussed it even with their children. Yet they confirmed the files. What an American clerk wrote in a converted hotel in 1945 matched exactly what the man himself remembered saying.
The Americans had been right about the ordinary aspects too. Much of the material in those reports was about home, weather, food, and rank, running for pages before anything useful to anyone appeared. The hotel outlived the operation by six decades. The Greek Orthodox Church bought it in the late 1940s and ran a monastery there for a time, after which it sat open to the weather and to anyone who wanted to write graffiti.
On July 25, 2005, embers from a small grass fire crept into the old building, and it burned along with two outbuildings before anyone could stop it. Park Service historians who later walked the ruins found acoustic insulation in the ceilings and remnants of microphone wiring inside the walls. That leaves us with the men who sat at the listening posts. Many of the Nisei linguists who sat in that ground-floor room transcribing and translating what Japanese prisoners said when they thought they were unheard had relatives behind the wire in American internment camps the entire time they did it.
Their parents had left the West Coast in 1942, the same year the Army was choosing Byron Hot Springs and calculating its acreage. The sons wore American military uniforms and held security clearances for one of the most secret facilities in the country. Their mothers were in camps at Manzanar, Topaz, and Tule Lake. They did not talk about that, and they did not talk about the nature of their work either.
One of them said long afterward that no order had been needed on that point, but knowing the place was highly secret was enough in itself to make everyone who served there, in his words, keep absolute silence. When the Park Service began its research in the early 2000s, about 20 of the 45 known men who worked at the two centers could be found, and eight Camp Tracy veterans were still alive. Most of them had told no one for 60 years, including their wives.
The men whose families were behind American wire were among the most secretive of all, and nothing in the Byron Hot Springs papers explains why.