US investigators claim to have found evidence of an assassination plot against 50 Cent, America's biggest selling music artist, allegedly hatched by a convicted drugs trader in revenge for some of the rapper's lyrics.

Court documents emerged yesterday that show federal investigators believed there was a possible conspiracy between the New York drug lord Kenneth "Supreme" McGriff and some employees of the music label Murder Inc to kill the rapper.

The star, whose real name is Curtis Jackson, survived nine gunshots in May 2000, but refused to co-operate with police.

The investigators have also examined whether the killing of Jam Master Jay, member of Run DMC, in 2002 was connected to the alleged feud between 50 Cent and McGriff.

An affidavit from Francis Mace, a special agent assigned to the US treasury department, discloses that law enforcement officials believe that the shooting in 2000 was in retaliation for the lyrics of the song Ghetto Koran. The song detailed the history of the Supreme Team, McGriff's gang. Mr Mace said that he believed there was an "ongoing plot" to kill 50 Cent and that McGriff was kept in touch with the singer's whereabouts via paged conversations with Murder Inc employees.

The document was written in 2003 and filed under an application to search Murder Inc's Manhattan offices.

The previously sealed search warrant was contained in a motion filed last week for Murder Inc's founder, Irving Lorenzo, who is charged with laundering more than $1m (£544,000) of McGriff's drug money.

Curtis was raised in a rough part of the Bronx and was orphaned at eight. He was selling crack on the streets by the time he was 12. But he went on to huge success. After signing with Eminem's record label, Shady Records, his debut album sold more than 10m copies. His latest album sold 1m copies in four days. Hollywood has taken note; the director Jim Sheridan is making a film of his life.

A lawyer who represented McGriff said he was aware of the affidavit, but that the allegations had disappeared in subsequent government documents. The US attorney's office in Brooklyn, which is leading the inquiry, had no comment.

McGriff, 45, was released from jail in 1995 after eight years, and prosecutors allege he immediately reformed his gang. He is currently serving a 37-month sentence for gun possession and is facing additional drug trafficking and murder charges. Jury selection is due to begin for Mr Lorenzo's trial next month.

After releasing the song Ghetto Koran, 50 Cent was blacklisted from studios in the US, said Mr Mace. Jam Master Jay acted as a mentor to 50 Cent and was killed in his recording studio. "Law enforcement agents are investigating the possibility that [he] was murdered for defying the blacklist of 50 Cent," Mr Mace wrote. The crime is unsolved.

The violence stemming from the hip-hop music scene has prompted New York police to set up a unit to monitor the business.

Other deaths have included Tupac Shakur and Notorious BIG.

• This article was amended on Monday 6 March 2009. In the article above we had the name of Kenneth "Supreme" McGriff as Keith. This has been amended.