"The investigation is done," FBI spokesman Mike Turner said Tuesday. "There's a report that's being prepared by the case agent."
The report on the reopened case is expected to be delivered before the end of the year to District Attorney Joyce Chiles of Greenville, who has said when she receives it she will decide whether to have a grand jury consider indictments.
Till, a black teenager from Chicago, was visiting relatives in Mississippi in August 1955 when he was tortured and killed, purportedly for whistling at a white woman.
An all-white jury acquitted Roy Bryant and his half brother, J.W. Milam, in the killing, and the defendants have since died. Many have considered Till's death and the subsequent trial a catalyst for starting the civil rights movement.
The U.S. Justice Department reopened the case last year, prompted in part by a documentary that found errors in the original investigation and concluded that several people, some still living, were involved in Till's abduction and killing.
Alvin Sykes of Kansas City, who heads the Emmett Till Justice Campaign, praised the FBI's work.
"We firmly hope and believe the completion of this FBI investigation represents a thorough and fair investigation that was promised to (Till's mother) before she passed," he said. "We've moved one step forward to finding justice and pursuing the truth."
Till's mother, Mamie Till-Mobley of Chicago, died in 2003 at age 81.