If you don't know much about the 2001 anthrax attacks on the US then the UCLA Department of Epidemiology has a nice website with lots of links.
The FBI website has a lot of images of original material and presents a fairly compelling case that U.S. Army Medical Research Institute of Infectious Disease researcher Bruce Ivins was solely responsible. However since Ivins killed himself in 2008 as the investigation closed in there was no trial and, for some people, no closure.
In response to this the FBI asked the National Academy of Sciences (NAS) to conduct an independent review of the scientific evidence that led the agency to implicate Ivins in the anthrax letter attacks. The NAS committee released its report three years later after exhaustively covering all the evidence and conducting public hearings. They concluded that it was "impossible to reach any definitive conclusion about the origins of the anthrax in the letters, based solely on the available scientific evidence". The report also challenged the FBI and U.S. Justice Department's conclusion that a single-spore batch of anthrax maintained by Ivins at his laboratory at Fort Detrick in Maryland was the parent material for the spores in the anthrax letter.
This conclusion was welcomed by various conspiracy theorists although it really only points out the weaknesses in the governments case. Of course the fact that the FBI had previously been convinced that someone else was the perpetrator doesn't exactly inspire confidence. Dr Steven Hatfill eventually sued the FBI, the Justice Department, John Ashcroft, Alberto Gonzales, and others for violating his constitutional rights and the government settled the case for $5.8 million.
Regardless of whether he was the perpetrator or not (my guess is that he was) Bruce Ivins was clearly a few sandwiches short of a full picnic which raises the question that everyone seems to overlook:
Why was a mentally disturbed person allowed to continue working with highly dangerous pathogens?
Answer: Because the army wants them to.
"Individuals will be mentally alert, mentally and emotionally stable, trustworthy, physically competent....."
This is part of the US army regulation covering scientists working on bioweapons. (Actually bioweapon 'defence' since the US doesn't work on bioweapons).
Unfortunately it appears that this modest list of conditions - which, let's face it, isn't that different to the characteristics you might look for in a date - is too much for the workers at the US Army Medical Research Institute of Infectious Diseases (USAMRIID). In 2007 the deputy commander for safety wrote:
'The possibility of losing talented and well-trained researchers to other facilities.... with less stringent programs.... may impact the ability of the Defense Threat Reduction Agency to provide research personnel to combat biological agent use.'