An anthrax-tainted letter sent to former Senate Majority Leader Tom Daschle in 2001. (Getty)

FBI investigators say the case of the 2001 anthrax attacks is closed, but a new paper by a group of leading scientists suggests the real killers were never found.

The anthrax attacks began a week after 9/11 and targeted political figures and members of the media. Envelopes containing powdered spores of the deadly disease were mailed to two Democratic Senators and the offices of NBC News, ABC News, CBS News, the New York Post and the National Enquirer.

In all, the attacks sickened 17 people and left five dead. Though the final letters were sent in early October 2001, they spawned several copycats and, for many months afterward, the possibility of anthrax tainted envelopes terrorized workers at media companies and political offices around the country.

Almost exactly seven years later, in August 2008, the FBI and Department of Justice announced that Dr. Bruce Ivins, a scientist at the United States Army Medical Research Institute of Infectious Diseases who worked on the investigation was their "sole suspect" and "the only person responsible for these attacks." Dr. Ivins committed suicide by overdosing on painkillers  days before the announcement after learning he was suspected of the attacks. Much of the evidence against Dr. Ivins was circumstantial and based on his strange behavior. He had an office near the location where some of the letters were postmarked, he kept odd hours, he was obsessed with a sorority and he stockpiled pictures of blindfolded women on his computer. The FBI also claimed genetic evidence linked the anthrax strains used in the attacks to a strain from Ivins' lab. Last February, the FBI and Justice Department officially declared the case closed in a 92 page report that said the investigation, code named "Amerithrax," conclusively "established that Dr. Ivins, alone, mailed the anthrax letters."

The new study, which was written by a trio of anthrax experts and will be published in the upcoming issue of the Journal of Bioterrorism & Biodefense, calls for the case to be reopened. It claims anthrax spores used in the attacks contained chemicals that indicate sophisticated manufacturing processes Dr. Ivins could not have carried out on his own. Critics of the paper interviewed by the New York Times speculate these chemicals could be meaningless contaminants rather than a smoking gun.

This new report is far from the first time serious questions were raised about the Amerithrax investigations. Over the years, multiple members of Congress and several Senators have expressed doubts about the case. Last February, the National Academy of Sciences questioned the strength of the case against Dr. Ivins following a 19 month review of the investigation. Specifically, the review faulted the FBI for exaggerating the strength of genetic evidence linking Dr. Ivins to the attacks.

Amerithrax cost taxpayers $15 million including a $5.82 million settlement paid to Dr. Steven Hatfill, who was initially identified by the government as a "person of interest" in the case and subsequently exonerated. The Government Accountability Office, Congress' audit, evaluation and investigative, is currently reviewing the evidence in the anthrax case.