A blitz by federal agents during the last three weeks captured nearly 2,100 illegal immigrants across the country in raids targeting child molesters, violent gang members and past deportees who re-entered the country.

U.S. Immigration and Customs Enforcement officials credited the roundup to a network of 35 fugitive apprehension teams.

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"This is a massive operation," said Marc Raimondi, a spokesman for immigration enforcement or ICE, which is part of the Department of Homeland Security. "We are watching the country's borders from the inside."

The crackdown, dubbed "Operation Return to Sender," kicked off May 26. An Associated Press reporter and photographer accompanied a fugitive task force as it made raids Tuesday night and early Wednesday.

Click here to watch a FOX News reporter's ride-along with ICE during the raids

A swarm of immigration agents had sped silently, headlights off, down a Boston side street and surrounded an apartment house.

"Police! Policia! Police!" Monico yelled, holding his badge to a window where someone had pulled back the curtain. "Open the door!"

Soon agents led a dazed-looking Jose Ferreira Da Silva, 35, out in handcuffs. The Brazilian had been arrested in 2002 and deported, but had slipped back into the country. He now faces up to 20 years in prison.

"This sends a message," said Daniel Monico, a deportation officer, after a successful raid early Wednesday. "When we deport you, we're serious."

The operation has caught more than 140 immigrants with convictions for sexual offenses against children; 367 known gang members, including street soldiers in the deadly Mara Salvatrucha, or MS-13; and about 640 people who had already been deported once, immigration officials said. The numbers include more than 720 arrests in California alone.

More than 800 people arrested already have been deported.

ICE's 2006 budget increased the number of fugitive task forces to 52, and the Bush administration is pushing for 70 by 2007. The teams face a mounting challenge.

There are more than 500,000 "fugitive aliens" who have been deported by judges and either slipped back into the country or never left. There is often a disconnect between local and state prisons and the federal government that allows illegal immigrants to serve time and be released without being transferred to federal officials for deportation.

The government has conducted large scale sweeps from time to time, including on April 20, when Homeland Security Secretary Michael Chertoff announced a new get-tough policy. That day, agents rounded up 1,100 illegal immigrants in 40 cities.

During the raid late Tuesday, the federal squad, which includes a Boston police sergeant detective, wore bulging bulletproof vests and stiff Kevlar gloves to protect their hands from needles, knives and rusty fences.

Badges dangled on chains around their necks as they passed around wanted posters and shined flashlights on the face of a 24-year-old Latvian man who had served prison time for assaulting a police officer.

The team moved in the dark, climbing fences and hiding behind parked cars to encircle a three-story house in Boston's Allston-Brighton neighborhood. All at once they emerged from the shadows. A half-dozen agents filled the front porch, their knocks on the door echoing down the block. The target had moved, the agents learned, and a team split off and caught him in Weymouth, about 15 miles south of the city.

Another man caught in the recent blitz was a Salvadoran gang member who was convicted in a stabbing that left a 13-year-old boy paralyzed. Agents caught him working at Budget Rental Car at Boston's Logan Airport.

"The problems with immigration aren't going to be solved overnight," Raimondi said as the team sped toward another raid. "You start chipping away at it ... The more teams we get up and running, the more dangerous people we are going to get off the streets."