Jesus F. Palacios was arrested last month after sheriff's deputies spotted him trying to jimmy a car door.
Using newly acquired access to federal immigration files, they scanned his fingerprints into a database at the police station and instantly determined the 30-year-old Mexican was a previously deported illegal immigrant with a criminal record.
Deputies alerted federal authorities and got an order to hold Palacios on suspicion of immigration violations. He had been removed from the U.S. at least three times after being convicted of child molestation and grand theft auto.
In the past, suspects like Palacios may have been freed on bail as deputies waited for hours for federal and state officials to do a background check.
But now, as part of a post-Sept. 11 crackdown, the Department of Homeland Security and the Justice Department are working to give local authorities rapid access to fingerprint data that can help determine if suspects under arrest are in this country illegally.
"It has been a tremendous advantage to local law enforcement," said Los Angeles County sheriff's Lt. Larry Bryant, who manages the database. "There would be a very good chance these people would be undetected and be released if we didn't have this capability."
The Los Angeles County Sheriff's Department and nearly 50 other Southern California police and law enforcement agencies have been given rapid access to federal records on criminal immigrants. So have the police in Boston and Dallas.
The U.S. government intends to make information available to all local law enforcement agencies in the nation within two years to help catch criminal immigrants, people with ties to terrorist groups, and others who pose a threat.
"We need more people on the ground with information who can act," said Robert Mocny, acting director of the Home Security visitor-technology program.
Civil rights advocates worry that enlisting local authorities could chill relations in communities where police rely on immigrants to come forward with information about crimes without fearing their own status will be checked.
"The No. 1 tool law enforcement uses to solve crimes is law enforcement-community cooperation," said John Trasvia, president of the Mexican American Legal Defense and Education Fund.
In Los Angeles County, Bryant said there is no indication officers have arrested people just to check their immigration status. "We've got better things to do," he said.
This year, federal authorities have placed immigration holds on more than 6,700 inmates in jails operated by the Sheriff's Department, according to the government.
Since police in Los Angeles County started using the database in September, they have identified and detained about 130 illegal immigrants with criminal records.
In most cases, local authorities cannot legally hold suspects longer than four hours before releasing them on bail or with a citation, Bryant said. However, it often took the FBI and state agencies longer than that to check a suspect's fingerprints and return results, he said.
The new database -- housed in the Sheriff's Department and accessible to other law enforcement agencies in the county -- cuts that time to minutes.
If there is a match, the agency contacts a federal law enforcement center in Burlington, Vt., to request an immigration hold. The suspect is turned over to federal authorities only after the local criminal case has been resolved.
Palacios faces charges of failing to register as a sex offender and using fake citizenship papers. He was not charged with trying to break into the car because it belonged to an acquaintance, authorities said.