Matthew Walker died on April 11 at the Charlotte County Correctional Institution.
Florida Department of Corrections

Published: Apr 22, 2014 5:47 PM EDT

A southwest Florida family is seeking justice for a beloved uncle and brother who died in prison.

Matthew Walker died April 11, in the Charlotte County Correctional Institution, but how he died remains a mystery.

Walker, 55, had served about 13 years of a 20-year sentence.  He was convicted of burglary and robbery in Palm Beach County.

"It's really, really hard.  No one is getting any rest," said Mae Atkins, Walker's sister.  "The only thing I was told was that he had died, and that was it."

Since then Atkins said all she has heard are rumors.

"To hear that he was handcuffed while he was beat to death -- oh my God.  This is just unbelievable," Atkins said.  "(Investigators) wanted to know if Matthew had complained about anything going on at the prison. They wanted to know, 'Had he complained about any specific  officer?'"

Atkins said, based on visitations with her brother and letters he had sent from prison, Walker did have ongoing issues with an officer.

"He did tell me something about some big bald-headed guy that didnt like him," Atkins said.

Atkins did not want to reveal the officer's name, but said her brother's stories were disturbing.

"(The officer) used the 'n-word'  and told (Walker) what he was going to do to him," Atkins said. "And he did exactly what he said he was going to do."

The Florida Department of Law Enforcement is investigating Walker's death.

Atkins said she plans to contact the NAACP.

The union representing the corrections officers told WINK News they would like to know about the staffing situation at the prison at the time Walker died.

"I can tell you, typically, within all the institutions, they're starting off at what they call 'critical level,' or below critical level.  That's the minimum amount of officers needed to safely do the job," said Les Cantrell, the statewide coordinator for Teamsters Local 2011.

Teamster coordinators said Charlotte County has one of the worst staffing problems in the state.

"We're definitely concerned any time that theyre working at or below critical level," Cantrell said.  "It puts the officers and the inmates at risk."