The Star Trek legend conducted a telephone interview with Lee Boyd Malvo, who is serving life in prison in Virginia for killing 10 people with his pal John Allen Muhammad in shooting sprees across Washington, D.C. in 2002.
And Shatner was left stunned when the convicted felon began opening up about their murders, admitting he and Muhammad, who was executed last year (09), had initially planned their killings with two other men.
Malvo told Shatner, "There were two other people who were supposed to be involved but in the end they ended up backing out.
"What was supposed to happen was that there was supposed to be three to four snipers with silenced weapons, silenced rifles and in this way you could do a lot more damage along the entire Eastern Seaboard... In the end there was only Muhammad and myself."
Montgomery County Police in Maryland have confirmed they will be questioning Malvo again over his part in the crimes.
And the 79-year-old actor, who filmed the chat for his new series Aftermath With William Shatner, insists the interview was an unforgettable moment in his career.
Shatner says, "Getting the opportunity to speak with Lee Malvo is a moment I'll never forget. He was only 17 when he committed these horrific acts, a monster forged by the only father figure in his life, and it was simply astonishing that he's found the maturity and humility to admit so many new crimes in his effort to make amends."