A record 157,000 illegal immigrants (search) were removed from the United States during the past year as U.S. authorities stepped up efforts to track down those who may pose security risks, the government said Tuesday.

About half of those deported in the year ending Sept. 30 had criminal convictions, an increase of 6.6 percent over the previous year, while removal of illegal immigrants who have no criminal record increased by more than 10 percent, Immigration and Customs Enforcement (search) officials said.

Much of the increase came from targeting fugitive immigrants who failed to report for removal after receiving an order for deportation.

The agency said it has pursued illegal immigrants more aggressively since it learned that terrorists involved in the Sept. 11 attacks exploited the immigration system

"We know that other dangerous criminals have sought illegal entry by similar means," said Michael J. Garcia, the Homeland Security Department's assistant secretary for immigration and customs enforcement. "We are bringing to bear the full force of our authorities to locate and remove those in the country illegally," he said in a statement.

Of the deported undocumented immigrants, the vast majority, 71 percent, were returned to Mexico, with Honduras, Guatemala, El Salvador and Brazil rounding out the top five countries.

Some immigration advocacy groups question whether more deportations will improve security.

"These immigration laws are being enforced against immigrants who are simply in this country to work and find a better life for themselves," said Michele Waslin, an immigration policy analyst at the National Council of La Raza (search), a Hispanic civil rights organization. "We don't have any evidence that they were national security threats or terrorists."

Most of the immigrants with criminal convictions were convicted of minor crimes such as theft and marijuana possession, Waslin said. Her group advocates letting illegal immigrants earn legal status if they work and pay taxes.