Francisco Galicia paced his cell at Fort Hunter Liggett, a vast army base 160 miles south of San Francisco, on a Friday evening in January. His mind raced with thoughts of his five daughters waiting for him at home.

Over several hours, immigration agents brought six more men into the frigid, cement-walled cell. As the men shared eerily similar stories of their arrests, Galicia realized they had all driven straight into a trap.

All seven had been driving home from fishing at a popular county lake when an official in a white truck had pulled them over along the same stretch of Jolon Road, a public, two-lane road that, unbeknown to Galicia, cuts through a corner of the military installation.

The traffic stops appeared routine at first: a light out, an open gas cap, a trunk door ajar, driving over the line. But then the officer asked for a social security number. In each case, when the men didn’t give a number, immigration agents arrived within minutes to make arrests, then drove the men to a detention site on the base where they were held overnight before being transferred to an immigration facility, Galicia said.

“Seven fishermen went fishing, and we’re the ones who got hooked,” Galicia, who was deported days later, said from his home town in the southern Mexican state of Guerrero.

The arrests of Galicia and his cellmates point to an apparent collaboration between Fort Hunter Liggett police and immigration agents that has ensnared at least 15 men since 30 December, according to Galicia and a local rapid response network, which confirmed eight additional cases. The arrests were first reported by the bilingual news magazine Voices of Monterey Bay.

Together, the accounts suggest that the Department of the Army civilian police at Fort Hunter Liggett – whose mission is to maintain law and order on base property – assisted in the federal government’s nationwide drive to arrest undocumented immigrants, a scheme that military law experts and members of Congress say may violate a US law restricting the use of the military on domestic soil.

A federal judge previously ruled the Trump administration violated the 19th-century law, the Posse Comitatus Act, when it deployed national guard troops to Los Angeles in June amid widespread protests against Trump’s increasingly unpopular deportation campaign.

The traffic stops and on-base detentions at Fort Hunter Liggett appear to violate the spirit of the act and represent a “creep in the wrong direction toward military participation in law enforcement”, said William Banks, a Syracuse University law professor emeritus and expert in the domestic role of the military.

The concerns spurred two members of Congress from California, Zoe Lofgren and Jimmy Panetta, to pen a letter to the homeland security secretary, Kristi Noem, and secretary of the army, Daniel Driscoll, in February, inquiring about Fort Hunter Liggett’s participation in immigration enforcement.

“We have received multiple, consistent reports indicating that FHL police have conducted roadside stops along Jolon Road and coordinated with Immigration and Customs Enforcement (ICE) in a manner suggesting racial profiling,” the letter states, citing “serious questions regarding jurisdiction, interagency coordination, statutory authority, and the protection of civil liberties”.

A high-stakes traffic stop

In recent months, Galicia increasingly avoided public spaces as he heard horror stories of people caught in the federal immigration dragnet, opting to spend time in nature instead.

On 16 January, after his shift as a tractor operator at a vineyard, he drove to Lake San Antonio, the passionate recreational fisherman’s favorite spot to cast a line or take his family camping.

After several quiet hours without catches, the 36-year-old resident of Greenfield packed up his gear, eager to get home to his daughters. All US citizens, the girls range in age from three to 15.

He drove along Jolon, passing the turnoff towards Fort Hunter Liggett’s gated entrance. At 165,000 acres, the base is the US army reserve’s largest training installation and includes vast wilderness areas open to permitted hunting on weekends and holidays.

In his rearview mirror, he spotted a white truck barrelling towards him at high speed. Fearing a drunk driver, he closed his eyes, bracing for impact. Instead, the truck slowed, then flashed police lights. Galicia pulled over in shock.

Galicia said the officer, dressed in a dark civilian police uniform, told Galicia a license plate light was out. He collected Galicia’s California driver’s license, insurance and car registration. A few minutes later, the officer returned to ask for Galicia’s social security number. Frightened, Galicia admitted he didn’t have one.

Within minutes, Galicia said, three vehicles surrounded his car and six immigration agents piled out, announcing he was under arrest.

Galicia’s account of the traffic stop is supported by an armed forces traffic ticket, which was among belongings returned to Galicia upon deportation. The pink form indicates Galicia was stopped for a “warning” at about 7pm along Jolon Road. Handwritten remarks state that “all installed lighting must be maintained in good working order”. The officer who issued the ticket identified himself with “FHL PD”, the acronym for Fort Hunter Liggett’s police department, and a single name: “Walker”.

The Monterey County Solidarity Rapid Response Network, a grassroots coalition that responds to local immigration activity, said it had received reports from family members of 11 people, including Galicia, who were arrested by immigration agents following traffic stops along Jolon Road, including two swept up the same night as Galicia. The network confirmed the account of at least one other man besides Galicia who was held on base.

In response to queries about the traffic stops and alleged cooperation with immigration agents, an army spokesperson said Fort Hunter Liggett did not disclose specific law enforcement operations and had not received any federal directives to work with ICE.

The spokesperson said that base law enforcement only asks for driver’s license, vehicle registration and insurance during routine safety and security stops, but people attempting to enter the cantonment area – the urban hub of the base with barracks, offices and support services – may be questioned about their immigration status and referred to an external law enforcement agency, which could lead to being temporarily held on the base.

Galicia said he and the other men detained that night were not driving towards or near the entrance gate to the base cantonment area, located on a road that turns off from Jolon. The army did not respond to follow-up questions about whether the Fort Hunter Liggett police broke policy by asking for social security numbers during traffic stops that were not near the gated entrance.

The Fort Hunter Liggett public affairs officer, Amy Phillips, declined an interview with the police department chief, but the office provided a map showing that base police share law enforcement jurisdiction with local agencies along the section of Jolon Road where Galicia said he was stopped. She said that if drivers fail to present proper documentation during a traffic stop, the base police may “do a hand-off”.

“If coordination with other law enforcement agencies is required, we will do so to ensure the proper agency assumes command of incident [sic] outside of our mission to protect Fort Hunter Liggett community and our military training mission,” Phillips said.

In California, undocumented immigrants can obtain state driver’s licenses, which are printed with the words “not acceptable for official federal purposes”.

‘A clear tell that this was subterfuge’’

The Posse Comitatus Act prohibits federal troops from acting as domestic police, unless authorized by statute or Congress.

Fort Hunter Liggett police are federal, civilian employees – not soldiers. But as Department of Defense employees, they are still restricted by the Posse Comitatus Act to only “engage in law enforcement for the primary military purpose of maintaining law and order on a military installation”, said Rachel VanLandingham, a retired air force lieutenant colonel and professor at Southwestern Law School.

VanLandingham said Department of Defense regulations, which apply to federal troops and civilian personnel such as Fort Hunter Liggett police, reinforce that interpretation. The rule prohibits actions primarily aimed at aiding civilian law enforcement or engaging in “subterfuge” to deceitfully bypass Posse Comitatus restrictions.

“Asking for a social security number … you don’t do that in a normal traffic stop. It’s a clear tell that this was subterfuge,” VanLandingham said, adding that the “triviality of the basis for the stops” showed they were “obviously just a pretext for pulling these folks over to check their documentation to check to see if they’re in this country lawfully”.

Banks, the Syracuse professor, said the traffic stops at Fort Hunter Liggett set a dangerous precedent of harnessing military resources for domestic political goals.

“This week it may be fingering migrants to ICE. Next week it could be picking up people with anti-Trump bumper stickers,” Banks said.

Immigration and Customs Enforcement and the Department of Homeland Security did not respond to a detailed summary of the arrests and the concerns raised by legal experts; an army spokesperson said the Department of the Army “does not comment on third-party accounts or external statements”.

Violations of the Posse Comitatus Act are federal crimes, but prosecutions are extraordinarily rare. Still, after a federal judge’s September finding that the national guard deployment in Los Angeles violated the act and his subsequent December order that they leave the city, the Trump administration withdrew remaining troops there. Troops were also quietly pulled from Chicago and Portland in January.

During his second term, Trump has blurred the lines between immigration enforcement and military duties. He has repeatedly threatened to invoke the Insurrection Act, a key exception to Posse Comitatus that allows deployment of troops to quell uprisings. The administration has expanded militarized zones along the southern border, empowering troops to apprehend migrants for trespassing, and erected a huge detention camp on Texas’s Fort Bliss army base.

Immigration arrests of people apprehended on military bases are not new, though they typically take place when people drive up to base entrance gates. Last May, Camp Pendleton Marine Corps base announced a security cooperation with ICE and CBP, and agents subsequently arrested at least five men in recent months who approached the entrance gate for on-base contract work or as ride-share drivers dropping off passengers, according to the San Diego immigration lawyer Valerie Sigamani, whose firm is representing the men.

In Ventura county, the 805 Rapid Response Network said it received reports of eight people arrested by immigration agents at the entrance gates at Point Mugu and Port Hueneme navy installations, after making a wrong turn. The network also said at least 20 cannabis workers arrested during large-scale immigration raids in July were temporarily detained on Point Mugu.

‘A very different world’

Since his arrest and deportation, Galicia is grappling with his new reality and separation from his family.

Galicia crossed into the US unauthorized and unaccompanied at the age of 16. For two decades, he sent several hundred dollars home each month. He bought his father a truck, horses, a water pump, a corn mill and a generator, gradually bringing economic stability to his parents, Indigenous Mixteco speakers from a rural pueblo.

Four days after his afternoon fishing trip, Galicia found himself back in Mexico, tearing up as he waited for a flight to Guerrero at the Tijuana airport. He recorded a TikTok video that would go viral, warning other undocumented fishers to stay away from Lake San Antonio and Jolon Road.

“Look for somewhere else to fish, but don’t use the areas that are military,” he said in the video. “They are cooperating 100% with immigration agents.”

Back in his home town, Galicia threw himself into building a home on a plot of land he bought several years ago. His wife, a stay-at-home mom, plans to bring the family to Guerrero when the school year ends. The two have depleted their savings, Galicia said.

“My daughters say they want to come, but it’s a world that’s very different from theirs,” Galicia said. “I pray to God that they can adapt.”

Galicia speaks to his family in California several times a day, showing them sun-drenched cornfields and coconut-laden palms. Only his three-year-old still struggles to understand why he doesn’t come home.

“She bursts into tears and tells me, ‘Daddy, you don’t love me any more, that’s why you don’t come.’” Galicia said. “No matter how tough you are, that’ll break you.”

This reporting was made possible in part by a grant in memory of and honoring the life and legacy of David Burnham, who was devoted to holding the government to account and Co-Founded the Transactional Records Access Clearinghouse (TRAC) in 1989