At 2:10 a.m., a fleet of dark SUVs surged from the garage beneath a federal building onto the deserted streets of Fairfax County, Virginia, carrying a raiding party of flak-jacketed immigration agents.

Their quarry: illegal immigrants who have ignored and evaded deportation orders. Called "fugitive aliens" or "alien absconders," they have nearly doubled in number since 2001, now totaling more than 636,000.

The Fairfax operation was part of a stepped-up national effort that has increased the number of fugitive arrests from 1,560 in 2003 to a projected 16,000 this year, U.S. immigration officials said.

"The absconder population is exhibit number one," said Victor X. Cerda, former chief of staff and general counsel for the Department of Homeland Security's Immigration and Customs Enforcement (ICE). "We haven't been able to handle the 600,000-plus who went through the legal system. What's going to lead us to believe we're going to handle the 12 million?"

Federal officials became alarmed after the Sept. 11, 2001, terrorist attacks when they discovered they could not account for 314,000 immigrants who had been ordered deported, including 5,046 from countries where al-Qaeda was present.

Since then, spending on fugitive operations has grown from $9 million to $183 million a year -- about $10,000 per arrest, according to a recent report by the Homeland Security inspector general. But the backlog continued to grow as immigration courts increased their workloads, issuing far more deportation orders.

Meanwhile, because of a shortage of detention space, many immigrants from nations other than Mexico who were caught sneaking across the border were freed in the United States to await their court dates -- a practice dubbed "catch and release." The vast majority never showed up for court, leading judges to order their deportation in absentia.

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