Squads of federal marshals and police rounded up hundreds of sexual predators and gang members in the eastern half of the United States as part of a nationwide manhunt that included 26 arrests in the city of Pittsburgh on Wednesday.

Deferring to the U.S. Justice Department, Pittsburgh police said they couldn't discuss the charges or provide any details.

But some of those caught here were sex offenders; others were fugitives with warrants for a variety of other felonies.

Attorney General Alberto Gonzalez will announce the national results on Thursday in Washington, D.C., and locally U.S. Attorney Mary Beth Buchanan plans a news conference to detail the arrests in the 25-county Western Pennsylvania district.

The roundup is the second part of a national effort to hunt down sex offenders. The first phase was in April, when the marshals arrested 1,102 fugitives on various sex offense charges in 27 states west of the Mississippi River.

A similar operation last year, which targeted mostly violent repeat offenders, netted 10,340 fugitives across the United States, the largest fugitive apprehension in history. In this region, more than 100 were captured.

This time the marshals and local police concentrated heavily on Megan's Law violators and other sexual predators, including those who have failed to register as sex offenders or are not living at the addresses where their Megan's Law form says they should be.

Keeping track of sex offenders has long been a headache for police who are often overburdened with other duties.

But this year a new law has designated the marshals service as the lead agency to hunt them down across the U.S.

The law, the Adam Walsh Child Protection and Safety Act, was signed by President Bush on July 27, the 25th anniversary of the abduction and murder of 6-year-old Adam Walsh in Florida.

Adam's father, John Walsh, is the host of "America's Most Wanted." He and his wife, Reve, attended the signing in the White House Rose Garden.

Under the law, convicted sex offenders who fail to register with local authorities will be charged with a felony in the U.S. District Courts. Previously, each state dealt with failure to register differently.

The first man in the country to be charged with the new federal crime of failure to register is John Henry Gill, 47, who was indicted last week in Salt Lake City.

The marshals' new initiative has parallels with the Justice Department crackdown on illegal guns under Project Safe Neighborhoods, in which the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives is the designated agency for pursuing ex-cons caught carrying weapons, a federal crime.

The marshals will assume a similar role for convicted sex offenders. Although he was not among those caught last week, one local example of the kind of fugitive the marshals will pursue is Larry M. Williams, 33, of Lincoln-Lemington, who was arrested at a North Side mosque this summer.

In addition to being wanted on federal weapons charges, he was also wanted for failure to register as a sex offender following his rape conviction in the state of Washington.

Since the Adam Walsh law went into affect, deputy marshals have been training with the National Center for Missing and Exploited Children to get up to speed on Megan's Law registry and child predator issues.