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.