The investigation found that Bruce Ivins acted alone in planning and executing these attacks.
As written here earlier, there is no doubt that Ivins was responsible for the letters and the deaths resulting from them.
Unfortunately, some people have been suckered into believing otherwise (within the pages of the Wall Street Journal no less), promulgating some sort of conspiracy theory, Sen. Pat Leahy among them.
The report concluded that the anthrax was not weaponized as some have written. Here is the excerpt from the report:
Throughout the course of the investigation, repeated challenges have been raised to this finding that the spores were not weaponized. The challenges have their root in an initial finding by the Armed Forces Institute of Pathology (“AFIP”) that, upon gross examination, the spores exhibited a silicon and oxygen signal. However, subsequent analysis of the spores by Sandia National Laboratories, using a more sensitive technology called transmission electron microscopy (“TEM”) – which enabled material characterization experts to focus its probe of the spores to the nanometer scale – determined that the silica was localized to the spore coat within the exosporium, an area inside the spore. In other words, it was incorporated into the cell as a natural part of the cell formation process. “The spores we examined lacked that fuzzy outer coating that would indicate they’d been weaponized,” stated Dr. Paul Kotula of Sandia, who personally examined the spores from the 2001 attacks. When presented with these results, Dr. Peter Jahrling, a USAMRIID scientist who had reviewed the initial AFIP results and stated publicly in late 2001 that the spores had been weaponized, retracted his earlier statement, telling the Los Angeles Times on September 16, 2008, “I believe I made an honest mistake.”
The report at the Justice link above makes note of new information about Ivins' strange habits which is interesting reading for those who need more reassurance that he was the man.
I suppose some will continue believing Ivins wasn't the culprit, but it serves us no purpose to ignore the truth and invent alternate realities.