The FBI investigated threats of violence made against Malcolm Glazer and his family around the time the late owner of the Tampa Bay Buccaneers was acquiring Manchester United, according to newly released documents.

Glazer, a self-made billionaire whose 2005 takeover of the English club was fiercely opposed by fans, died in May at age 85.

The FBI released more than 120 pages of partially redacted records, including details on threatening telephone calls and emails, in response to a Freedom of Information Act request.

Glazer had already owned the Bucs for a decade when he acquired Manchester United in a leveraged buyout valued at roughly $1.4bn. Fans who opposed foreign ownership decried the move, fearing he would sell off the stadium, raise prices and saddle the club with debt. At the time, some Manchester United supporters burned Glazer’s likeness in effigy, urged fans to wear black and wave black flags, and called for a boycott of the club’s sponsors. British police had also said they were investigating threats from a militant fan group.

On the morning of 12 May2005, the day Glazer became the majority shareholder, a male caller with a British accent called the Buccaneers headquarters, asked many questions about the Bucs and ended the call by saying he wished Glazer were dead, according to an FBI report on the threat.

Several hours later, a male caller again speaking with a British accent identified himself as a member of the Manchester United Action Group and warned that the Glazer family would be in danger unless Glazer backed down from his takeover bid, the FBI report said.

Another call came the following day, with the man saying he was the same person who had called before and asking, “Do you remember me?” The caller said he knew people who “were ready to take action against Glazer,” and against another person whose name is redacted.

The caller said, “I am giving you another warning” and set a deadline for Glazer to abandon his takeover bid. The FBI issued subpoenas as part of the investigation. But the records show that the case was closed without prosecution because the authorities could not conclusively identify the source of the threats.

Because of the threats, the NFL planned for security representatives for all teams the Buccaneers played that season to have pictures of members of the Glazer family, according to the FBI documents.

Within a year of the leveraged buyout, Glazer had two strokes and his children ran the club.