Police said nine illegal immigrants and three smuggling suspects were held in this Phoenix raid in April, adding to the immigration caseload. ROSS D. FRANKLIN / THE ASSOCIATED PRESS

FRESNO, Calif. - Federal prosecutions of immigrants soared to new levels this spring, as the Obama administration continued an aggressive enforcement strategy championed under President George W. Bush, according to a new study released Thursday.

The 4,145 cases referred to federal prosecutors in March and April were the most for any two-month stretch since Immigration and Customs Enforcement was created five years ago, the Syracuse University-based Transactional Records Access Clearinghouse found. They ranged from misdemeanor illegal entry to prosecutions of immigrants with criminal records.

The government's heavy focus on immigration investigations already is creating a heavy burden for the swamped courts along the U.S.-Mexico border, whose judges handle hundreds more cases than most of their counterparts in the rest of the country.

Federal authorities say that workload would grow if Arizona's controversial new immigration law were implemented. The new law requires police, while enforcing other laws, to check the immigration status of anyone they have a reasonable suspicion is in the country illegally. It will take effect July 29 unless blocked by a court.

"People already are working 10- or 12-hour days and on weekends to just meet the caseload," said Matt Dykman, a U.S. District Court clerk for the district of New Mexico in Albuquerque, where the percentage of cases referred by customs and the Border Patrol increased 54 percent from February to April this year.

The Department of Homeland Security, which oversees both agencies, did not immediately respond to a request for comment on the Syracuse findings.

Some of the increase may be due to seasonal upticks in the flow of migrants, who often tend to cross the border in time for the summer harvest or other temporary work, Dykman said.

The nonprofit academic research group obtained the latest figures from the Department of Justice under the Freedom of Information Act. That agency also declined to comment on the findings.

U.S. attorneys along the Southwest border, from Texas to Calfornia, handle the bulk of cases referred by the Border Patrol.

Department of Homeland Security figures show that the number of illegal immigrants in the country has fallen in recent years.

As of January 2009, an estimated 10.8 million people were in the country illegally, 1 million fewer than the 2007 peak, the department said.

"People already are working 10- or 12-hour days and on weekends to just meet the caseload."

Matt Dykman,

U.S. District Court clerk for the district of New Mexico in Albuquerque