The anthrax letters were sent to lawmakers and news organizations as the nation reeled in the aftermath of the Sept. 11, 2001, terror attacks.
The person informed of the decision to close the case was not authorized to speak about it before an official announcement expected later Friday, and therefore spoke on condition of anonymity.
The anthrax case was one of the most vexing and costly investigations in U.S. history until officials announced in 2008 that the lone suspect was Dr. Bruce Ivins, who killed himself as authorities prepared to indict him. The move Friday seals that preliminary investigative conclusion.
Investigators had been on the verge of closing the case last year but government lawyers decided to conduct a further review of what evidence could be shared with the public, according to several people familiar with the case.
Officials were hesitant about releasing some information because of concerns about violating privacy rights and grand jury secrecy, said those familiar with the case, who spoke on condition of anonymity because they were not authorized to discuss internal deliberations.
Laced with anthrax, the letters were sent with childish, blocky handwriting and chilling scientific expertise.
The spores killed five people: Two postal workers in Washington, D.C., a New York City hospital worker, a Florida photo editor and a 94-year-old Connecticut woman who had no known contact with any of the poisoned letters. Seventeen other people were sickened.
For years, the FBI chased leads.
Authorities tried to build a case against biowarfare expert Steven Hatfill, but ultimately had to pay him a multimillion-dollar settlement.
Then, last year, they announced that the mystery had been solved, but the suspect was dead.
Authorities said that in the days before the mailings, Ivins had logged unusual hours alone in his lab at the Army Medical Research Institute of Infectious Diseases at Fort Detrick, Md. They also say he threw investigators off his trail by supplying false leads as he ostensibly tried to help them find the killer.
As the FBI closed in on Ivins last summer, the 62-year-old microbiologist took a fatal overdose of Tylenol, dying on July 29, 2008. After Ivins' suicide, FBI Director Robert Mueller said the investigation found Ivins was the culprit, and prosecutors said they were confident he acted alone.
Skeptics - including prominent lawmakers - pointed to the bureau's long, misguided pursuit of Hatfill, and noted there was no evidence suggesting Ivins was ever in New Jersey when the letters were mailed there.
At the urging of lawmakers, the National Academy of Sciences has launched a formal review of the FBI's scientific methods in tracing the particular strain of anthrax used in the mailings to samples Ivins had at his Fort Detrick lab.