It's happened before, the FBI fumbling high-profile cases.

Now, it turns out that the FBI also dropped the ball on the Oklahoma City bombing, the largest terrorist attack in U.S. history, by failing to turn over files and physical evidence to Timothy McVeigh's attorneys.

That disclosure prompted Attorney General John Ashcroft on Friday to delay the convicted bomber's execution, scheduled for Wednesday, until June 11.

In recent years, the bureau failed to notice a Russian spy within its ranks, accused the wrong man in the bombing at the Atlanta Olympics and botched its investigation of a government scientist who handled nuclear weapons secrets.

With word of the new misstep, President Bush and Ashcroft were evasive when asked whether they still had confidence in the FBI. "I'm obviously concerned about an incident where documents have been misplaced. But I withhold judgment until I find out the full facts," Bush said at a news conference Friday.

Less circumspect was Sen. Chuck Grassley, R-Iowa: "We must change the FBI culture that has caused these colossal mistakes," Grassley, a senior member of the Judiciary Committee, said in a statement.

The committee will review nominees to replace FBI Director Louis Freeh, who is retiring in June.

"I want to know that the next FBI director is committed to sweeping changes," Grassley said.

Danny Coulson, a lead investigator with the FBI's hostage rescue team who took McVeigh into federal custody, said the incident creates a perception problem for the bureau. But, he added, "I'm sure there's nothing (in the documents) that changes the outcome of the case."

The FBI blames the problem on a computer glitch. Grassley is not so sure.

"We saw documents suddenly disappear in the Waco, TWA Flight 800 and Wen Ho Lee cases," Grassley said. "FBI careers are made in high-profile cases, and this is the fourth time in recent years where evidence has belatedly appeared. We have to be careful that withholding evidence is not done simply to win a case."

Kris Kolesnik, director of the National Whistleblower Center, a Washington-based nonprofit public interest organization, said the recent blunders reflect what he contended was the FBI's tendency to emphasize public relations over pure science or good investigative technique.

"The culture is driven by image -- don't embarrass the bureau, make the bureau look good," he said.

On the positive side, State Department officials are praising the way the FBI was able to penetrate a spy ring that Cuba had operated in Florida. Five alleged spies are on trial in Miami on charges of espionage and of involvement in the 1996 MiG attack on a Miami-based unarmed plane north of Cuba. Four Cuban-Americans were killed in the incident.

But success stories like this have been overshadowed by a series of missteps:

--In February, Robert Philip Hanssen, a 20-year agent at the FBI, was accused of selling national secrets to Moscow. Hanssen carried on his alleged spying activities for 15 years without being detected by his bosses.

--Joseph Salvati of Boston spent 30 years in prison for a murder he did not commit even though the FBI had evidence of his innocence. Salvati was freed in January after a judge concluded that FBI agents hid testimony that would have cleared Salvati because they wanted to protect an informant.

--Last year, the FBI botched an investigation of Wen Ho Lee, the Los Alamos National Laboratory scientist who was indicted on 59 criminal counts of mishandling nuclear weapons secrets. Lee spent nine months in solitary confinement. All but one count was eventually dropped.

--In 1999, the General Accounting Office said a report by the Bureau of Alcohol Tobacco and Firearms, which pointed early on to the explosion of a center fuel tank as the cause of the 1996 crash of TWA Flight 800, was never forwarded to the National Transportation Safety Board. The ATF provided the report to the FBI, but the FBI never sent it the safety board, the GAO said.

--The FBI targeted Richard Jewell in the bombing at the 1996 Summer Games that killed one person and injured more than 100 others. Jewell was cleared three months later.

--In the mid-1990s, the FBI suffered an embarrassing investigation of its world-renowned crime lab. Justice Inspector General Michael Bromwich criticized the lab for flawed scientific work and inaccurate, pro-prosecution testimony in major cases, including the Oklahoma City bombing.

--In July, former Missouri Sen. John Danforth said an FBI lawyer "goofed" in not telling superiors in 1996 that federal agents fired pyrotechnic tear gas canisters into David Koresh's Branch Davidian compound in Waco, Texas. Four agents and more than 80 Davidians died during a 51-day standoff with federal officials.

--During a 1992 standoff in Ruby Ridge, Idaho, an FBI agent fatally shot white separatist Randy Weaver's wife, Vicki, while she held her 10-month-old baby.