STARKE, Fla. -- Using gruesome autopsy photographs, a medical examiner described the injuries to death row inmate Frank Valdes on the witness stand Friday and concluded the prisoner was stomped to death.

William Hamilton, the district medical examiner, testified he believed Valdes' injuries, including 30 rib fractures, caused the inmate to lapse into unconsciousness and die within minutes.

"There was a total collapse of the chest cage," Hamilton said.

Hamilton, who performed one of the two autopsies on Valdes (pictured, right), testified Friday in the second-degree murder trial of four former prison guards. They stand accused of the July 17, 1999 fatal beating of Valdes, a death row inmate housed on X-wing at Florida State Prison.

Capt. Timothy Thornton, 36; Sgt. Jason Griffis, 28; Sgt. Charles Brown, 28; and Sgt. Andrew Lewis, 31; are charged with second-degree murder, aggravated battery and conspiracy to commit aggravated battery on an inmate.

"In my opinion, the cause of Frank Valdes' death was multiple traumatic injuries due to a beating," Hamilton said.

Under cross examination by defense attorneys, Hamilton conceded that some of the injuries could have been caused by Valdes flinging himself off the bars of his cell unto the concrete floor of his X-wing cell at Florida State Prison.

"I don't believe you can go in a self-destructive manner long enough to inflict that many injuries," Hamilton said.

Boot prints were found on Valdes' neck, chest, abdomen and back.

"You don't get boot prints when you fall against a stationary object or structure," Hamilton said.

The medical examiner also conceded that guards could have broken some of Valdes' ribs when they performed CPR, but added he did not believe all the ribs were broken by resuscitation efforts.

Hamilton testified that most of Valdes' fatal injuries happened a few minutes before his death.

Determining when Valdes' injuries occurred may be critical to both the defense and prosecution.

Defense attorneys claim their clients were involved in a violent cell removal earlier in the day, but they claim Valdes received only minor injuries and was checked out at the clinic.

They claim Valdes could have died when he was under the care of three X-wing corrections officers -- Montrez Lucas, Dewey Beck, and Donald Stanford. They will be tried later on charges of second-degree murder.

Prosecutors also believe Valdes was fatally injured in a second beating later in the day, after he had been examined and cleared by the prison medical staff.