The Justice Department has called into question a key pillar of the FBI's case against Bruce Ivins, the Army scientist accused of mailing the anthrax-laced letters that killed five people and terrorized Congress a decade ago. Shortly after Ivins committed suicide in 2008, federal investigators announced that they had identified him as the mass murderer who sent the letters to members of Congress and the media. The case was circumstantial, with federal officials arguing that the scientist had the means, motive and opportunity to make the deadly powder at a U.S. Army research facility at Fort Detrick, in Frederick, Md. On July 15, however, Justice Department lawyers acknowledged in court papers that the sealed area in Ivins' lab -- the so-called hot suite -- did not contain the equipment needed to turn liquid anthrax into the refined powder that floated through congressional buildings and post offices in the fall of 2001. The government's statements deepen the questions about the case against Ivins. Searches of his car and home in 2007 found no anthrax spores, and the FBI's eight-year, $100 million investigation never proved he mailed the letters or identified another location where he might have secretly dried the anthrax into an easily inhaled powder.