The detentions, which in some cases have nearly led to the deportation of citizens or legal residents, are drawing increased attention.

By Andrew Becker and Patrick J. McDonnell

Reporting from Tacoma, Wash., and Los Angeles — Rennison Vern Castillo thought his legal troubles were nearly over at the end of a jail stay for harassing his ex-girlfriend. But then a U.S. immigration hold order blocked his release.

"They think you're here illegally," a jailhouse guard said to him.

Castillo, mystified, insisted it was all a mistake. Though born in Belize, he had come of age in South Los Angeles, spoke fluent English, served a stint in the Army and had become an American citizen about seven years earlier.

He had some legal problems, but being in the country unlawfully was not one of them. Castillo said he wasn't worried -- not until he was shackled and transferred to a federal detention center. He spent months in custody before an appeals panel blocked his deportation and an immigration judge finally ordered Castillo set free.

Although his case is an extreme example, mistaken detentions are drawing increased attention as immigration officials mount workplace roundups and jailhouse sweeps in search of undocumented immigrants.

Immigration raids of factories and other work sites often result in at least a short-term detention of lawful residents and even citizens, as agents seal targeted businesses and grill workers about their status.

Officials in Washington said last month that the Obama administration was expected to rein in the controversial workplace raids -- shifting enforcement emphasis to target employers rather than workers. Immigrant advocates have long pushed for such a change, while others say easing workplace enforcement will encourage illegal immigration.

Castillo is one of many citizens and legal residents held for suspected immigration violations -- some for a few hours, some for much longer. No agency tracks such incidents, so statistical totals are not available.

Officials at U.S. Immigration and Customs Enforcement downplay the problem.

"ICE does not detain United States citizens," said spokesman Richard Rocha, adding that agents thoroughly investigated people's claims of citizenship. "ICE only processes an individual for removal when all available facts indicate that the person is an alien."

He declined to comment on Castillo's case or others, citing privacy concerns or pending lawsuits.

The surge in ICE workplace actions during the Bush administration spawned fierce complaints from employees caught up in dragnets at factories, slaughterhouses and poultry farms.

Mike Graves, a two-decade veteran of the Swift & Co. meatpacking plant in Marshalltown, Iowa, said he was handcuffed and held for eight hours in December 2006 when ICE agents raided Swift plants throughout the heartland.

"My government treated me like a criminal, and I didn't do anything wrong," said Graves, a native of Iowa.

An ICE raid last year at a Van Nuys printer cartridge manufacturer, Micro Solutions Enterprises, generated wrongful-arrest claims from more than 100 citizens, said Peter Schey, chief lawyer at the Center for Human Rights and Constitutional Law in Los Angeles. All were held for two to three hours before being released, Schey said.

Americans seldom carry proof of their legal status, which can be a factor in the confusion about detainees' citizenship. There is no comprehensive database or list of all citizens for agents to check.

Official investigations may miss crucial documents such as birth certificates and naturalization papers. In some cases, names have been jumbled or misfiled and records lost. Confused detainees have signed their own removal orders. Some in custody may even be unaware of their citizenship or unable to prove it without a lawyer's help.

Unlike suspects in criminal matters, however, immigration detainees have no right to government-appointed counsel -- and, in some cases, have no access to paid lawyers. Fast-track deportation procedures enacted by Congress in recent years also limit court review once the expulsion process is underway.

In border regions like Southern California, residents on both sides of the international boundary have for generations moved back and forth without regard for passports, status or birth certificates. Many U.S. citizens by birth or parentage have no proof of their status.

Frank Ponce de Leon, a native of Mexico who lives in La Puente, got out of ICE custody Dec. 31 after spending almost three months locked up -- all the while insisting he was a citizen. The longtime California resident had never sought citizenship because he was the son of an American-born parent. His father was a New Mexico native and U.S. serviceman during World War II.

"I knew they couldn't hold me forever, and sooner or later they would see it my way because I had every right," said Ponce de Leon, 47, whose five California-born children include a daughter, Deanne, 22, who served in Iraq as an Army nurse.

On occasion, the uncertainty can lead to mistaken deportation, as was the case with Pedro Guzman, a mentally disabled U.S. citizen living in Lancaster.

U.S. immigration officials shipped Guzman to Tijuana in May 2007 from the Men's Central Jail in downtown Los Angeles, where he was being held on a misdemeanor trespassing charge. The Los Angeles native, then 29, spent three months rummaging for food in dumps and sleeping in the Mexican borderlands as his desperate mother, a fast-food cook, searched for him in hospitals, shelters, jails and morgues, his family said.

Eventually Guzman, a cement finisher with limited Spanish and a second-grade reading ability, was reunited with his family in the border town of Calexico.

The Guzman case sparked Washington hearings at which immigration authorities were chastised by Congress members and accused of "stunning incompetence." ICE officials called the case an aberration and vowed to review all citizenship claims before anyone was detained or deported.

Out of more than 1 million detentions, ICE officials say, Guzman was the only citizen known to have been shipped out of the country. But others dispute that claim.

Rachel E. Rosenbloom, supervising attorney at Boston College's Post-Deportation Human Rights Project, cited at least eight cases of wrongly deported citizens and said she expected the number was substantially higher.

One such case, detailed in an upcoming report by Rosenbloom's group, is the curious saga of Duarnis Perez. He is a native of the Dominican Republic who became a U.S. citizen at 15 when his mother was naturalized. But he didn't know citizenship had been conferred on him as well. He assumed he was illegal, and so did everyone else.

Perez was deported and subsequently arrested trying to sneak back into the United States from Canada. He spent almost five years in prison for unlawful reentry. It was only upon his release in 2004 that an ICE official reviewed his file and informed Perez that he had been a citizen all along.

In Castillo's case, he was an infant when his mother left Belize and sought work in Los Angeles. She later became a nurse and sent for her son. Castillo attended elementary school in South L.A. and graduated from Thomas Jefferson High School in 1996. He became a naturalized citizen in 1998. He joined the Army and served in Korea, then was posted to Ft. Lewis, Wash. He was honorably discharged in 2003.

After domestic disputes with a girlfriend, he was convicted in 2005 of felony harassment and violating a no-contact order, and was sent to Pierce County Jail in Washington state for eight months. He was in a holding area with inmates about to be released when a corrections officer held him back.

Castillo was handcuffed and whisked off in a van to the Northwest Detention Center in Tacoma. A federal officer said records showed he was an illegal immigrant.

"Your records are wrong," Castillo said he replied. He said he told the officer that he was a citizen but that his naturalization certificate had never arrived. It was sent to the wrong address, he later learned.

Castillo went before an immigration judge, who appeared via video conference, a common procedure in the crowded immigration court system. Again, he claimed citizenship. The judge didn't believe him. He was ordered deported on Jan. 24, 2006.

The nonprofit Northwest Immigrant Rights Project, a legal advocacy center based in Seattle, provided a lawyer to handle Castillo's appeal. The lawyer searched for Castillo's naturalization documents and records of his military service.

The Board of Immigration Appeals blocked Castillo's deportation, noting proof of his military service. A month later, he was released without further explanation. It turned out Castillo was the victim of a paperwork mix-up: His name was spelled wrong in immigration records. And he had been assigned more than one "alien number," causing further confusion.

Castillo, now 31, is still incredulous.

"If it had taken 30 days to figure it out, I wouldn't be upset. But seven months?" he said in an interview.

He, like Guzman and others with similar experiences, has filed suit against the ICE.

"I want them to recognize they made a mistake," Castillo said. "Something needs to change. If it can happen to me, it's going to happen to someone else."

This story was reported and written in collaboration with the Center for Investigative Reporting in Berkeley, a nonprofit news organization. Andrew Becker is a CIR staff reporter. Patrick J. McDonnell is a Times staff writer.