San Diego ? The U.S. 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 U.S. 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. The 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 quickly investigate criminal histories.

“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 U.S. and committed more crimes. One of them was Rafael Resendez-Ramirez, a drifter who went 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.