FBI seeks to counter doubts on its anthrax investigation

WASHINGTON (CNN) - FBI officials offered more evidence Monday to support their assertion that government scientist Bruce Ivins was responsible for the anthrax-laced mailings that killed five people in 2001.

In a two-hour briefing at the agency's headquarters, senior FBI scientists - backed by a panel of outside experts - revealed they examined more than 1,000 anthrax samples and interviewed all of the approximately 100 people who had had access to the flask that reportedly held the deadly strain implicated in the killings.

In the briefing, intended to counter widespread questions about the investigation, the scientists fiercely defended the forensic process that led to their conclusion, but acknowledged that they may never be able to satisfy everyone.

"We'll never put all the questions to rest," said Vahid Majidi, head of the FBI's Weapons of Mass Destruction Directorate. "There's always going to be a spore on a grassy knoll."