In October 2001 U.S. federal and state authorities announced that letters containing Bacillus anthracis (anthrax) spores had been mailed to media outlets and political offices, causing multiple cases of anthrax infection and triggering a major public-health and law-enforcement response. The first reported cases emerged in early October; on October 15, 2001, officials confirmed the pattern of deliberate mailings after positive laboratory tests linked anthrax in postal letters to illnesses among postal workers, journalists and others. Scope and impact The attacks produced both inhalational and cutaneous anthrax cases. Recipients and mail handlers developed varying disease severity, with several deaths reported. The attacks disrupted mail service, particularly in the nation's capital, and led to large-scale distribution of antibiotics and prophylaxis to potentially exposed postal and government employees. Hospitals and public-health agencies mobilized to identify and treat exposed individuals, decontaminate affected facilities, and advise the public about symptoms and prevention. Federal investigation and response The events triggered a joint federal investigation led by the Federal Bureau of Investigation with public-health support from the Centers for Disease Control and Prevention (CDC). Investigators worked to trace the origin of the mailed spores, analyze the contaminated letters and environmental samples, and establish distribution timelines. The investigation also examined potential links to domestic or foreign actors and assessed whether additional contaminated mailings had occurred. Laboratory and decontamination work Laboratories across the federal system performed microbiological and genetic tests to confirm the presence of Bacillus anthracis in letters and environmental swabs. Contaminated sites—including postal facilities and targeted offices—underwent extensive cleaning and decontamination processes that involved hazardous-materials teams and specialized remediation techniques. These efforts aimed to reduce further exposure risk and restore safe operations. Public health messaging and medical countermeasures Public-health officials issued guidance on recognition of anthrax symptoms, recommended steps for possible exposures, and protocols for antibiotic prophylaxis. Hospitals enhanced infection-control measures and clinicians were advised on diagnostic testing and treatment regimens appropriate for inhalational and cutaneous anthrax. The events underscored the need for rapid laboratory capacity, clear risk communication, and coordination among public-health, medical and law-enforcement agencies. Legacy and investigations The anthrax mailings in October 2001 had lasting effects on postal operations, biosecurity practices, and emergency preparedness planning in the United States. They prompted reviews of mail screening, facility design, and cross-agency coordination in responses to biological threats. Over subsequent years federal investigations examined forensic and scientific evidence to identify suspects and assess motive. Some investigative conclusions and details became subjects of public and legal scrutiny, and aspects of the case remained complex and debated in later accounts. Historical context These biothreat incidents occurred in the weeks following the September 11, 2001, terrorist attacks, a period of heightened national anxiety. The overlap in timing intensified public concern and drew sustained media and governmental attention. The response combined public-health interventions with criminal investigation procedures, reflecting the dual nature of deliberate biological threats as both health and security challenges. This account draws on contemporaneous public-health and law-enforcement reporting from October 2001 and subsequent government summaries. Where authorities later revised findings or where legal and scientific debate persisted, those complexities were noted in official records and reporting.