Some prisoners who are legal U.S. residents have remained locked up for many months because they can't produce the documents to prove their identity, hampered by the fact that immigration detainees don't have constitutional protections such as the right to counsel, The San Francisco Chronicle reported Monday.

Advocates say some who find themselves in the situation were born in other countries and acquired citizenship through a U.S.-born parent, or through a parent who became a naturalized citizen. The American Civil Liberties Union Immigrants Rights Project says others have mental health problems and frequently they are poor.

"These are people who are the most vulnerable," said Judy Rabinovitz, the project's deputy director. "People are being locked up without bond hearings, often for long periods."

"The Constitution is the same that applies to U.S.-born citizens as to naturalized citizens," Sin Yen Ling of San Francisco's Asian Law Caucus told the Chronicle. "Detaining these folks is creating a third category of people with a different set of rights."