The actors first met while filming a 1964 episode of TV hit The Man From U.N.C.L.E., and they reconnected two years later when they were both cast in the original Star Trek drama series.

The stars famously endured a rocky relationship early on in their careers, but they later grew close and became good pals as they teamed up for appearances at Star Trek fan conventions around the world.

The two men reportedly fell out after Nimoy, who portrayed Mr. Spock, accused Shatner of filming him without permission for his 2011 documentary The Captains, for which he interviewed the captains of the Starship Enterprise.

"I thought he was kidding," Shatner wrote in his new book, Leonard: My Fifty-Year Friendship With a Remarkable Man. "It was such a small thing."

The old pals never resolved their conflict before Nimoy passed away last year (Feb15) from complications of chronic obstructive pulmonary disease (COPD), at the age of 83.

"We had some disagreements to begin with and in the end, but in between, it was a brotherhood," Shatner, who played Captain Kirk, explained on U.S. breakfast show Today. "I loved him."

However, Shatner now insists he isn't completely clear as to why Nimoy turned his back on him and snub his numerous attempts to patch up their differences.

"I don't know," Shatner told Today as he was quizzed about what led to the friendship fall out. "That's the mystery that will go to everybody's grave. I swear to God... He wouldn't talk to me..."

When asked by co-host Kathie Lee Gifford if Nimoy perhaps suffered from dementia, Shatner refused to speculate and said, "I don't wanna go there because his family's around him... (It's) heartbreaking."

Shatner, 84, also revealed he made one final attempt to make amends with Nimoy, but he has no idea if the late actor found it in his heart to forgive him before he passed away.

"I sent him a note before he died of love, friendship, and how I missed him," he continued. "I never got a response."