The FBI has decided with finality that a government researcher acted alone in the deadly 2001 anthrax mailings and is closing its long-running investigation, a person familiar with the case said Friday.

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.