Government agents pile into unmarked sport-utility vehicles and dark sedans in the basement garage of downtown's federal building. Their assignment: Crash the homes of illegal immigrants and deport them.

It is an increasingly common scene across the nation. The federal government wants to catch the nearly half-million immigrants who either have ducked deportation orders or are targets for removal because they have been convicted of a crime.

The size and complexity of the mission is staggering. Even as the government pours millions of dollars into enforcement, each year the number of new fugitives far exceeds the number of immigrants removed.

One spring evening in San Diego shows why.

The agents were eager to start knocking on doors: Each wore a bulletproof vest with "POLICE" emblazoned on the back or a blue Department of Homeland Security jacket. But only four were full-time agents with Fugitive Operations, a unit founded in 2002 to track these immigrants; the other nine either worked days processing deportation orders or were Border Patrol agents on overtime. The cobbled-together teams had five hours to finish the job.

By night's end, they had apprehended six of their 16 targets, with a seventh picked up the next morning. Given that the teams fan out just several times a week, it was barely a dent in the region's backlog of 5,000 cases.

Orders from Washington are to pursue violent criminals, a small fraction of all the fugitives, but the San Diego agents catch whomever they can. Four of their seven had convictions unrelated to immigration -- for crimes including battery, theft, sex with a minor and drunken driving.

Alvina Martinez, 54, a homemaker whose husband works in construction, had no such record. Martinez was deported in 1998 for being in the United States illegally; a second offense would make her a felon. Agents talked their way into her small, single-story home and deported her to Mexico the next day.

San Diego has one of 18 Fugitive Operations teams and, with more than 550 apprehensions, ranks near the top of the 22 cities where Homeland Security agents have caught fugitives since October. The others include Los Angeles, Boston, Miami and Chicago.

In all, Homeland Security wants to round up about 460,000 fugitive immigrants, about 80,000 of whom have criminal records unrelated to immigration. In May, the Associated Press asked for a database with details about these fugitives, but Homeland Security has not ruled on that Freedom of Information Act request.

Authorities hope to eliminate their backlog by 2009.

Federal agents will have detained nearly 10,000 fugitives during the fiscal year that will end on Sept. 30. It is impossible to know how many of those deported have already returned to the United States. During the same period, an estimated 40,000 new fugitives were added -- so the list has actually grown longer.

The explanation is straightforward.

Homeland Security has about 19,500 detention beds nationwide. Although local jails hold some of the overflow, overwhelmed immigration courts often release immigrants who are challenging their deportation and trust they will show up for hearings.

Some do. And many who are captured and threatened with deportation voluntarily return home. But agents acknowledge it is not surprising many skip hearings that likely would lead to their removal.

In April, the department expanded a pilot program to jail immigrants while their cases wind their way through the courts. That effort, begun in Connecticut and expanded to Atlanta and Denver, has drawn criticism from immigration lawyers who say it punishes noncriminals who are simply exercising their right to due process.

Authorities are also experimenting with new ways to track people. In June, Homeland Security began using electronic ankle bracelets in eight cities, including Denver, San Francisco and Portland, Ore. The department launched a limited program under which immigrants out on bail or on parole check in by telephone, with voice-recognition software verifying they are who they say they are and that they are calling from home.

When immigrants do go on the lam, Fugitive Operations agents must pursue "this population that has been out there flouting the law," said Victor Cerda, who oversees detention and removals at Homeland Security's Immigration and Customs Enforcement division.

Agents spend hours building dossiers: staking out homes to determine when best to come knocking, interviewing apartment managers, checking credit reports and loan applications. Some agents rely on ruses to enter people's homes, knowing they are unlikely to be allowed in if they explain their true intentions. It is called "knock and talk."

Homeland Security's $4 billion spending plan for fiscal 2005 seeks $69 million for Fugitive Operations, a fourfold increase from the $17 million this fiscal year. The department wants to expand the number of squads, each typically with five members, from 18 to 48 nationally. Homeland Security officials said that, as a matter of policy, they do not reveal where those squads are stationed.

Agents used to track the freshest cases first, figuring they would be easiest to find, said Doris Meissner, President Bill Clinton's top immigration official.

"I think it is, by and large, a losing battle to go out and try to find people," said Meissner, now senior fellow at the Migration Policy Institute in Washington.

The top priority has become getting felons -- not those with simple immigration law violations.

Homeland Security officials say felons account for 47 percent of the fugitives removed this year, short of their goal of 70 percent. About 11 percent of apprehended fugitives have committed sex offenses or other violent crimes, and about 15 percent were drug offenders.

Some critics say the government relies too much on enforcement instead of addressing the fundamental reasons immigrants come. In some cases, the critics say, the government is stoking anti-immigrant sentiment.

"By trying to characterize undocumented immigrants as criminals, it makes it easier to scapegoat them," said Gail Pendleton, associate director of an immigration project at the National Lawyer's Guild.

Jaime Garcia Zuniga, 23, is the kind of criminal Homeland Security wants to deport. The Mexican, who has been convicted of fighting in public and of having sex with a minor, was perhaps the biggest catch for the San Diego agents that evening.

While agents were chatting with someone who answered the door, Garcia removed a screen and sneaked out through the window of his first-floor apartment.

"We've got a runner!" one agent shouted.

Agents cornered Garcia in an alley, where he surrendered.

The agents returned to their cars, still catching their breath -- one down, more than 400,000 to go.

Immigration agents from the Department of Homeland Security search a suspected illegal immigrant in San Diego. U.S. Immigration and Customs Enforcement agents in San Diego are briefed before being dispatched to serve warrants on illegal immigrants.Immigration agents search a home in San Diego in a bid to capture illegal immigrants who have defied deportation orders.