After years of false leads, the FBI increasingly focused on Bruce Edwards Ivins, a scientist at the U.S. government's biodefense laboratories at Fort Detrick in Frederick, Maryland. Ivins was placed under periodic surveillance in 2007, and investigators built a case around the fact that the anthrax in the letters belonged to the Ames strain and appeared genetically related to material in a flask from his laboratory. On July 29, 2008, before he could be charged, Ivins died from an overdose of acetaminophen in what authorities determined was a suicide. Federal prosecutors nevertheless declared him the sole perpetrator on August 6, 2008, citing DNA evidence and a combination of circumstantial and behavioral evidence. The conclusion immediately became controversial. The National Academy of Sciences, reviewing the FBI's scientific methods in a 2011 report, agreed that the strain identification was sound but found insufficient scientific evidence to establish that the letter material originated specifically from Ivins's lab. Senators Chuck Grassley and Patrick Leahy called for scrutiny of the investigation, while the FBI maintained that science was only one part of a broader case. The bureau formally closed Amerithrax in 2010, but the absence of a trial, sealed evidence, and continuing questions about Ivins's mental health left the official solution disputed rather than universally accepted.
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