Immediately after the Sept. 11, 2001, attacks on the World Trade Center and Pentagon, the mission of the nation's federal prosecutors urgently shifted to focus on catching and stopping terrorists before they could strike again.

Now the U.S. attorney's office in Washington is creating a special unit to meet the increasing demands brought on by a spike in cases involving national security. U.S. Attorney Kenneth L. Wainstein said he will devote about 12 prosecutors full time to the effort.

Wainstein and his fellow prosecutors in Washington are shouldering a rising caseload of people suspected of threatening or planning a terrorist attack, trafficking in dangerous weapons and helping people sneak into the country illegally. Recent cases have involved such defendants as Lowell W. Timmers, the Michigan man sentenced for falsely threatening to blow up the White House with his van, and Wasam Delaema, a Dutch citizen charged last month with aiding insurgent attacks in Iraq.

The creation of the "national security section" reflects the office's high priority on anti-terrorism cases. The office already is at the forefront of the investigation of the 2001 anthrax attacks. Wainstein said he hopes to build more expertise within his office and forge stronger partnerships in sophisticated cases involving diplomatic and intelligence agencies.

Besides taking on more responsibility for terror cases in the United States, Wainstein wants his office to get more involved in prosecuting those accused of terrorist acts against U.S. citizens and overseas interests.

In recent years, federal law enforcement agencies have turned to the Eastern District of Virginia and the Southern District of New York to prosecute some of the most significant terrorism cases, though the Washington office would have seemed a natural choice.

"U.S. attorney's offices have to earn these cases, and we've kicked it up a level where we've earned them," Wainstein said. "We just have to keep doing what we're doing. If you get into the cases early and do a good job, the next time law enforcement agent Jones has a big case, he calls Assistant U.S. Attorney Smith."

Wainstein's announcement mirrors the restructuring efforts taking place at the Justice Department and FBI. Other U.S. attorneys also are devoting more resources to national security.

In Washington, the national security section will bring together prosecutors handling cases involving terrorism, terrorism hoaxes, espionage, leaks of classified information and other sensitive matters. It will include roughly 10 prosecutors who already had been spending much of their time on national security cases, plus two or three more prosecutors to be added later.

The section will be led by Matthew G. Olsen, a veteran prosecutor and supervisor in the office who spent the last 15 months working on a detail as special counsel to FBI Director Robert S. Mueller III. Olsen returns to the U.S. attorney's office in September.

Olsen said he had a "front-row seat" at the FBI to observe the uniqueness of national security cases. Prosecutors may spend years tailing a subject, then be forced to give up on a possible conviction to glean information that prevents a terrorist attack.

"What I saw at the FBI is that these cases take time to develop," Olsen said. "And they end up in a prosecutor's office because of relationships with a law enforcement agency . . . trust that develops over years."

Olsen is a longtime friend of Wainstein, and his career has repeatedly dovetailed with Wainstein's. They attended the University of Virginia at the same time, clerked for federal judges in U.S. District Court in Washington after graduation and overlapped for a few years in the 1990s when both worked as front-line prosecutors at the U.S. attorney's office.

Wainstein also had worked at the FBI for Mueller, serving as his chief of staff. Wainstein recommended Olsen as a candidate for the FBI detail job in June 2004. Just days later, Wainstein learned that the White House was going to nominate him to be the acting head of the prosecutor's office. President Bush later nominated him to be U.S. attorney.

In tandem with the creation of the national security section, Wainstein announced a change meant to address federal judges' concerns about the spotty performance and supervision of prosecutors rotating between assignments in D.C. Superior Court and U.S. District Court. Most of the prosecutors in the rotation work on narcotics and weapons cases in federal court but get very limited exposure to other, more complicated cases.

Wainstein said he plans to give rotating prosecutors an opportunity to work on a larger variety of cases and to get additional supervisors to help them. He hopes that will allow the office -- now slightly below national average for its number of white-collar prosecutions -- to build expertise in prosecuting white-collar crime and other fraud cases.

U.S. Attorney Kenneth L. Wainstein, left, said he will devote about 12 full-time prosecutors to the special terrorism unit, including Matthew G. Olsen, who will be its leader.