New reports have claimed that late singer Robin Gibb was investigated by the FBI after allegedly making death threats against his first wife’s divorce lawyer.

Robin Gibb separated from Molly Hullis in 1980 after 12 years of marriage and newly-released documents show just how ugly the split turned.

Staff at Haymon & Walters, the law firm representing Molly Hullis, reported Robin Gibb to authorities after receiving “numerous threatening telegrams from Gibb which threatened their lives”, the records show.

He told them he had hired a hitman to kill them, sparking a probe by investigators that could have led to jail.

Robin Gibb, who died from colon cancer in May, sent telegrams from his home in Miami, Florida to the law firm of Haymon & Walters.

In one, Robin Gibb wrote: “What you have done is just about the limit. I warned and warned you. The situation is now very serious. Know one (sic) walks all over me… I have had enough.”

He added: “I have taken out a contract on (name deleted by FBI). It is now a question of time.”

Robin Gibb was investigated by FBI after making death threats to ex-wife Molly Hullis' lawyer

The New York Post acquired the legal papers from the FBI after a Freedom of Information request following Robin Gibb’s death in May after a battle with liver and colon cancer.

They show that investigators considered taking further action and asking Western Union, who sent the telegrams, for further information.

Agents also discussed how to look into the allegations without tipping off Robin Gibb who was at the height of his fame with brothers Maurice and Robin after the success of the album Saturday Night Fever.

The matter was eventually dropped after Robin Gibb’s lawyers wrote to the FBI and said the singer “would not be foolish enough to carry out any threat, especially in view of his singing career”.

According to the New York Post, Robin Gibb’s accused his wife of tipping off the FBI in an attempt to embarrass the star as a negotiating tactic in their divorce.

Documents reveal that the FBI closed the file on Robin Gibb in March 1981 because Molly Hullis and her lawyers declined to press charges.

A spokesman for Gibb family said they had no comment to make.

Robin Gibb and Molly Hullis, who met when he was just 18, had two children by the time he was in his early 20s. But the marriage had began to fall apart when he became dependent on amphetamines to stay up recording.

In interviews before his death, he revealed he did not spend as much time with her as she wanted.

“She wanted more of a home and roots. Because of my nature and work, I needed to keep changing my environment,” he said.

As she raised their children in Surrey, England, he went on a one-night-stand rampage across the U.S., estimating he slept with more than 100 women.

“They were mostly a distraction, almost like notches on a belt,” he recalled.

After their bitter divorce and custody battle – where he was banned from seeing his children, Molly Hullis remained in the UK after their divorce while the Gibb brothers settled in Florida.

Robin Gibb married his second wife, artist and author Dwina Murphy, in 1985 and they went on to have a son. In 2008, his housekeeper Claire Yang gave birth to their love child.

Robin Gibb once revealed he and Dwina, who were married until his death, had an “open marriage” and she confirmed: “We have an open relationship. Robin has had flings with friends of mine and he talks to me about them.”