Two cousins accused of killing a teenager during her visit to Oregon acknowledged entering the country illegally from Mexico, a federal Immigration and Customs Enforcement official said.

One of the men, Alejandro Rivera Gamboa, 24, went unnoticed by immigration officials even though he was arrested four times on drunken driving charges since 2000, The Oregonian newspaper reported.

He and 23-year-old Gilberto Arellano Gamboa were arrested this week in the death of 15-year-old Dani Countryman of Kaufman, Texas, whose body was found late last month in an apartment southeast of Portland.

Oregon law prohibits local police from actively searching out illegal immigrants, but the rules change when foreigners land in jail. If local authorities find reason to believe the person is deportable, they may notify Immigration and Customs Enforcement.

At the same time, immigration officials search local jails and flag inmates who may be in the country illegally.

"We can't cover every jail in the United States," said Lorie Dankers, an immigration spokeswoman, noting that her agency also relies on local law enforcement officials to notify the immigration agency when they get inmates with questionable residency status.

In Rivera Gamboa's case, immigration officials have no record of contact after arrests, according to the Clackamas County Sheriff's Office, in 2000, 2003, and twice in 2006.

After their arrests in the murder case, the cousins told a federal immigration officer they have been in the United States illegally for about six months, Dankers said. According to state records, Rivera Gamboa obtained an Oregon identification card in April 2006.

In Clackamas County, where hundreds of jail inmates are released early each month because of overcrowding, illegal immigrants could be booked and released before immigration authorities ever get a chance to speak with them. It is unclear how much time Rivera Gamboa spent in the Clackamas County Jail, but a misdemeanor charge of drunken driving likely wouldn't have kept him there long.

Lt. Mike Alexander, Clackamas County jail operations manager, said local law enforcement officials are also limited in how much they can pursue federal immigration questions.

"If (inmates) choose not to tell us, we don't have any protocols that allow us to go forward," Alexander said.