Former Boston mob boss James "Whitey" Bulger has been found guilty of murder and racketeering in 31 out of 32 criminal charges against him.

But jurors found that federal prosecutors had proven their case against Bulger in only 11 of the 19 murders he was accused of carrying out or ordering in his days as head of the "Winter Hill" gang in the 1970s and 1980s.

The jury reached its verdict on its fifth day of deliberations in the US District Court in Boston.

The Irish-American mob leader faces the possibility of life in prison.

The 83-year-old had pleaded not guilty to all charges.

However, his lawyers had acknowledged that their client was a drug dealer, extortionist and loan shark, in short an "organised criminal".

During the two month-long trial, the 12 jurors heard vivid descriptions of Bulger's crimes.

They included daylight shootings of fellow criminals, the extortion of a victim at whose crotch Bulger aimed a machine gun and how one associate would pull teeth from the mouths of dead victims, hoping to make the bodies harder to identify.

His Irish links were well-known, particularly his pivotal role in shipping arms to the IRA in the 1980s.

Only a tip-off to the Irish and American authorities prevented tonnes of weapons adding to the IRA arsenals.

But the tip-off led to the murder of John McIntyre, another one of the gruesome deaths that Bulger was found to be connected to.

The most complicated count the jury ruled on was the second racketeering offence, which encompassed 38 criminal acts including all 19 murders Bulger was charged with.

The jury found that the government had proved its case on 11 of those murders, that it had not proven its case on seven and reached no finding on one.

The jury only needed to find Bulger guilty of committing two of those 38 crimes, which also included extortion, drug dealing and money laundering, for him to be guilty of racketeering.

The jury reached no finding in the death of one woman, Debra Davis, a girlfriend of Bulger’s associate Stephen "The Rifleman" Flemmi.

Flemmi testified at trial that Bulger strangled Davis in a south Boston house after she learned of their dealings with the FBI.

Other witnesses had testified that Flemmi himself finished Davis off.

The government's case relied heavily on three former top associates of Bulger - Flemmi, John "The Executioner" Martorano and Kevin Weeks.

Bulger had declined to testify on his own behalf.

Family members of Bulger's victims have long waited for verdicts on the killings, and about a half-dozen survivors have been a regular presence in the courtroom.

Bulger fled Boston in 1994 after a tip from a corrupt agent that his arrest was imminent.

He spent 16 years on the run, many of them on the FBI's Ten Most Wanted list, before agents caught up with him in June 2011, living in a seaside Santa Monica, California apartment.

He will be sentenced on 13 November.