STARKE, Fla. -- Three former prison guards charged in the murder of a death row inmate were found not guilty of all charges Friday.

Capt. Timothy Thornton, 36, and Sgts. J.D. Griffis and Charles Brown, both 28, were charged with second-degree murder in the July 17, 1999, slaying of inmate Frank Valdes at Florida State Prison.

During the closing arguments, defense attorneys showed photos of other inmates who had testified for the state against the guards. The attorneys said that their character represented reasonable doubt.

The jury of five men and one woman deliberated for 3 1/2 hours before returning the verdict.

The guards were also accused of conspiracy to commit aggravated battery, battery of an inmate and official misconduct. Thornton faced an additional charge of accessory after the fact.

Valdes was on death row for the 1987 murder of a corrections officer in West Palm Beach.

Prosecutors alleged Valdes was beaten to keep him from talking to reporters about mistreatment of inmates.

The attorneys for the guards argued that some of the five corrections officers to be tried later could have been responsible for killing Valdes (pictured, file photo at left). They also argued some of Valdes' injuries were caused by his climbing the bars in his cell and throwing himself onto the floor and his bunk.

Defense attorneys also said some of his ribs may have been broken when guards were performing CPR.

Two medical examiners who examined Valdes' body determined he was stomped to death.