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About 55,000 immigrants in the U.S. both legally and illegally were in federal prison last year, a 7 percent increase from 2005, government auditors said Thursday.

But the percentage of immigrants charged with federal crimes has been stable over the same period, at about a quarter of the federal prison population, the Government Accountability Office found. The majority of jailed immigrants are Mexican citizens.

The GAO estimates that the federal government spends $1.5 billion to $1.6 billion a year to incarcerate this population in federal and local facilities. The Department of Justice reimbursed state and local officials for jailing about 296,000 immigrants for crimes and civil violations in 2009,a jump of 35 percent over 2003. Prisons in California, Arizona, New York, Florida and Texas jailed the largest number of immigrants, a reflection of those states’ large influxes of newcomers.

In one of the biggest changes in recent years, immigrants arrested for crimes by the Immigration and Customs Enforcement agency are now being deported at sharply increasing rates: In 2007, 8,877 immigrants were deported after their convictions, compared to 78,690 in 2010, the GAO reported.

Based on a random sample, the GAO estimates that immigrants charged with crimes were arrested seven times on average, and 65 percent were charged at least once with an immigration offense.

Immigration violations, drug and traffic offenses accounted for about half the arrests, auditors found.

In other news,Federal Diary columnist Joe Davidson reports on progress on the government’s hiring reform effort, the Post’s politics team analyzes the sooner-than-expected resignation of Sen. Ensign and Al Kamen is In the Loop on Sen. Harry Reid’s secretive delegation to China.