The Daily Mail reports that the singer, who separated from Hullis after 12 years of marriage, was investigated by the US authorities following allegations that he made numerous death threats to his wife’s London-based lawyers.
The documents, which were released last week in the US by the FBI under Freedom of Information laws, reportedly show just how ugly the split turned.
Robin Gibb of the Bee Gees at his wedding to Molly Hullis in 1968
In a telegram to solicitors Haymon & Walters in March 1981, the newspaper reports he claimed he had hired a hitman. "I warned and warned you," he cabled. "The situation is now very serious.
"Know [sic] one walks all over me . . . I have had enough. I have taken out a contract. It is now a question of time."
Last month it was reported Gibb, who died from cancer in May, refused to visit doctors for scans that could have detected his illness because he wanted to go on a world tour.
Gibb, 62, died after a long battle with colon and liver cancer.
His widow Dwina Gibb, 59, told the Daily Mail Weekend magazine that the music star initially ignored her pleas and doctors' advice to have the cancerous cells fully checked.
The Bee Gees' song catalogue, which includes Massachusetts, I've Gotta Get A Message To You, Lonely Days, How Can You Mend A Broken Heart, How Deep Is Your Love and Stayin' Alive, led to their induction into both the Rock and Roll Hall of Fame and the Songwriters Hall of Fame.
Gibb's twin brother Maurice died of a heart attack in 2003 following intestinal surgery and Gibb's younger brother Andy, who was not part of the Bee Gees but a successful singer in his own right, died in 1988 from heart failure aged 30.