Feverish and delirious, Bob Stevens arrived at a Florida hospital in the early morning hours of October 2, 2001. The emergency room doctors thought the 62-year-old photojournalist might be suffering from meningitis.

But when an infectious disease specialist looked at Stevens’ spinal fluid under a microscope, he realized there was another, terrifying possibility. Lab tests confirmed it, and on October 4 Stevens was diagnosed with inhalation anthrax, a bacterial disease primarily found in livestock that the Center for Disease Control (CDC) had recognized as a potential agent of bioterrorism.

Over the next two months, Stevens and four other people would die after inhaling anthrax, and 17 others would be infected, either by inhaling anthrax or getting it on their skin. The lethal spores arrived via a series of letters mailed to locations in four states (Florida, New York, New Jersey and Connecticut) and Washington, D.C., spreading a new wave of panic across a nation already reeling from the terrorist attacks of 9/11 just weeks earlier.

After anthrax was discovered at Stevens’ workplace, American Media, and two more of his colleagues of his were found to have been exposed, state authorities in Florida (including the state’s then-governor, Jeb Bush) initially tried to calm the public down by insisting there was no terror link.

“The first day or two following that announcement were words of denial that this would have been related to any terrorism event, but it was somehow a freak outbreak of some sort,” says Leonard Cole, an expert on bioterrorism and terror medicine and author of The Anthrax Letters. “But as more information came out...it became clear that there had been some conscious, deliberate release of anthrax.”

The FBI launched an investigation, and by early November had found three of the letters containing anthrax spores, including ones sent to the offices of Senate Majority Leader Tom Daschle in Washington, D.C., and The New York Post and NBC in New York City. The public furor only intensified after law enforcement authorities determined that the first group of anthrax-laced letters had been posted from a mailbox in New Jersey on September 18, 2001—just a week after the 9/11 attacks.

A second bunch of letters had been mailed on October 9. In addition to anthrax powder, some of the letters also contained threatening notes using radical Islamic rhetoric, including such phrases as “Death to America. Death to Israel. Allah is Great.”

Concern about bioterrorism had ramped up in the years prior to the anthrax attacks, with increased awareness of and training in dealing with anthrax and other “select agents,” or those biological agents (bacteria, fungi, viruses) known to be potential weapons. This increased focus helped health officials respond more effectively to the attacks than they might otherwise have been able to do. In early 2002, President Bush announced he would be requesting some $11 billion over the next two years to further protect the nation from bioterrorism.

Meanwhile, the FBI continued their investigation, focusing by mid-2002 on Dr. Steven Hatfill, a scientist who had once worked in the U.S. Army’s Medical Research Institute of Infectious Diseases (USAMRIID) at Fort Detrick, Maryland, which kept stocks of anthrax. Identified as a “person of interest” in the attacks, Hatfill steadfastly maintained his innocence and was eventually cleared. (He would successfully sue the U.S. Justice Department for defamation, winning a $5.8 million settlement in 2008.)

FBI investigators then zeroed in on another scientist, Dr. Bruce E. Ivins, who also worked at USAMRIID and had been trying to develop a more effective vaccine against anthrax. Placed under 24-hour surveillance and banned from the labs where he had spent some 30 years, Ivins was hospitalized for depression and anxiety, and in July 2008 committed suicide by taking a lethal dose of acetaminophen (Tylenol).

In February 2010, the FBI closed its investigation into the anthrax attacks after more than eight years, issuing a 92-page report that concluded that Ivins carried out the attacks entirely on his own. But the results of the investigation were later called into question by the National Academy of Sciences, which issued a study in 2011 determining that while the scientific evidence supports the idea that Ivins might have been the perpetrator, it does not prove it conclusively.

In the wake of that study, a 2014 report by the Governmental Accountability Office (GAO) found that the scientific methods (including genetic data) the FBI used to identify Ivins as the culprit were flawed, casting further doubt on its conclusions.

If the anthrax attacks of 2001 sharpened the nation’s focus on protecting itself against future acts of bioterrorism, that focus faltered in the decades that followed, says Cole. Immediately after the attacks, “we were on a rocketing upwards of concern, funding and preparedness and general awareness,” Cole says. “But as is part of human nature, I guess...the more distant you are from the actual time of the events, the less people will be concerned about it.”

Prepared or not, the threat of another biological attack remains real. In late September 2018, letters were mailed to the Pentagon that reportedly contain ricin, a poison extracted from caster beans that has been used in previous bioterrorist attacks.