An already controversial Obama administration push to find and arrest illegal immigrants is facing a new round of criticism from civil rights and immigrant groups, who say that documented U.S. citizens are being detained under the program. How often is this really happening? Here, a brief guide to the federal government's immigration crackdown, and some of its unforeseen consequences:

Are immigration agents really detaining U.S. citizens?
Yes, at least in a flurry of recent cases. These people are typically picked up by local police for other reasons — U.S. citizen Antonio Montejano of Los Angeles, for example, was arrested for allegedly shoplifting a $10 bottle of perfume last month. But in custody, he was tagged as a suspected illegal immigrant by federal agents, acting on bad information in their database. As is typical in these cases, Montejano was supposed to be released quickly, but immigration agents told police to hold him in a local jail for possible deportation.

Wait, they can do that?
In theory, it's OK to hold people who have been arrested if they're genuinely suspected of being in the U.S. illegally. Under the controversial Secure Communities program, federal agents can tell police to keep criminals locked up, pending an investigation into the detainees' legal right to be in the country. But mistakes happen: Montejano spent two extra days in jail after the judge in his shoplifting case ordered him released. "I was telling every officer I'm an American citizen," he tells the Los Angeles Times. "Nobody believed me."

Could Montejano take legal action?
Possibly. "Any case where an American is held, even briefly, for immigration investigation is a potential wrongful arrest," says Julia Preston in The New York Times. The Immigration and Customs Enforcement agency (ICE), which runs Secure Communities, takes the allegations "very seriously," spokesman John Morton tells the Times. "We don't have the power to detain citizens," he says.

How often is this happening?
That depends on whom you ask. The American Civil Liberties Union points to the cases of Montejano and three others as irrefutable evidence that the problem is real, and widespread. The ACLU cites a study which found that 82 people were held — for up to a year — at Arizona immigration detention centers until a judge said they couldn't be deported because they were citizens. But the Center for Immigration Studies, a think tank dedicated to reducing immigration, says the ICE database "contains no records of U.S. citizens who were detained by or for ICE."

Sources: Center for Immigration Studies, LA Times, NY Times