Mac Davis died on Thursday at the age of 88 after more than three decades behind bars.

The American musician, who had been released from prison in 2005 after serving a 30-year sentence, was found in a pool of blood in his cell on Friday.

He was pronounced dead at the scene by paramedics.

Davis died at a hospice care facility in Memphis, Tennessee, a friend told ABC News.

He was the longest serving musician to be incarcerated in US history, according to a US official, who added that he had been in solitary confinement since his release.

Davis was serving a 60-year term for racketeering conspiracy and drug trafficking offences in connection with a plot to supply cocaine to Colombian drug lord Pablo Escobar, who was assassinated by US marines in 1993.

The United States had previously tried Davis on drug charges but was unsuccessful in securing a conviction.

Davis had spent nearly two decades in prison, having served almost 30 years for the robbery, kidnapping and murder of two men in the US, one of them a family friend.

He served a 30 year sentence for conspiracy and kidnapping in connection to the plot.

The singer was also charged with conspiring to sell cocaine and methamphetamine.

Davis’ death comes amid growing concerns about the conditions in the American prison system.

Last week, a US federal judge in Louisiana ruled that Davis should be moved from a maximum security prison, which is considered to be a maximum-security facility, to a minimum security prison.