SAN DIEGO — The Border Patrol has arrested tens of thousands of people with criminal records, including suspected murderers, rapists and child molesters, since the agency last year installed a fingerprinting system that identifies criminals among the 1 million illegal migrants apprehended annually.

The high-tech system is part of a broader effort by the Department of Homeland Security to create a "virtual border" to stop terrorists and those with criminal pasts from entering the United States.

The fingerprints of all detained illegal immigrants are now matched against the FBI's national criminal database through scanners installed at all 137 Border Patrol stations along the Mexican and Canadian borders. To process a person, all 10 fingerprints are rolled across a scanner, and the digitized images are compared against the database's 47 million records. Results usually come back within minutes.

About 30,000 of the 680,000 illegal migrants who were arrested from May through December were identified as having criminal records, compared with about 2,600 during the same period in 2002 — more than an eleven-fold increase. Criminal illegal immigrants are those with past arrests or convictions for crimes ranging from shoplifting to murder.

Since its start as a pilot program in 2003, the system has identified about 24 people suspected of homicide, 55 of rape and 225 of assault, according to Border Patrol statistics.

The system — installed over a six-month period ending in September — has made it difficult for suspects to flee the country and then return. That was common in the past when illegal border-crossers who had criminal records or outstanding warrants often were simply deported because agents lacked tools to investigate criminal histories quickly.

"You never knew who the people were who you arrested," said Dale Landers, a supervisory agent who patrols the back country east of San Diego. "This guy might look like someone who works in the fields, but he could have been a suspected killer."

Some suspects re-entered the United States and committed more crimes. One was Rafael Resendez-Ramirez, a drifter who had gone on a murder spree in Texas, Illinois and Kentucky and was captured and released by border agents in 1999 despite his presence on the FBI's most-wanted list. He went on to kill four more people before turning himself in.

The surge in arrests probably will strain the ability of federal agencies to house and prosecute criminal illegal immigrants, law-enforcement experts say.

How the Border Patrol handles the people it identifies depends on their records. People who have active warrants against them are handed over to agencies that issued the warrants. Those with violent-criminal records can be prosecuted for illegally re-entering the country and face potential 20-year prison terms.

People stopped at the border who have prior convictions for nonviolent crimes — the majority of cases — usually are expelled from the country, according to Border Patrol officials.

The technology overhaul, experts say, has enhanced border policing greatly. "It's a great step forward ... a great aid to law enforcement," said Joseph King, a professor at John Jay College of Criminal Justice in New York.

The apprehensions provide a potential bright spot for Homeland Security. The department has been criticized for being slow to take advantage of new technologies that confirm the identities of people entering the United States.

Similar systems have been installed at many U.S. ports of entry and airports, where a small percentage of visitors are screened. Homeland Security eventually wants to scan the fingerprints of all foreign visitors.