WASHINGTON — FBI scientists early on had — but destroyed — the unique strain of anthrax used in the deadly 2001 attacks that years later would lead them to Dr. Bruce Ivins, the government’s top suspect in the nation’s biggest bioterror case.

FBI Assistant Director Vahid Majidi said yesterday the initial anthrax sample that Ivins took from his Army lab in February 2002 and gave investigators did not meet court-ordered conditions for its preparation and collection.

In a briefing for reporters, Mr. Majidi said the sample kept at the FBI lab was destroyed because the bureau believed it might not have been allowed as evidence at trial.

“Looking at hindsight, obviously we would do things differently today,” Mr. Majidi said.

Ivins, 62, took a fatal dose of acetaminophen last month as prosecutors prepared to indict him for murder.

The anthrax letters killed five and sickened 17 after turning up on Capitol Hill, in newsrooms and postal facilities. Among those who died was 94-year-old Ottilie Lundgren of Oxford, Conn.

Ivins gave investigators a second sample of anthrax from his lab in April 2002 to comply with standards in a subpoena issued in the case. But that sample contained a different strain than what he submitted two months earlier in what prosecutors call an attempt to deceive or confuse investigators.

Mr. Majidi, who heads the FBI office in charge of investigating weapons of mass destruction, led a panel of government and private-sector scientists who detailed the scientific case against Ivins. They credited new ways of looking at the DNA of anthrax to whittle the list of labs and suspects who could have produced it.