PANAMA CITY, Fla. - The medical examiner who performed the first autopsy on a 14-year-old boy who died after an altercation with boot camp guards told jurors Wednesday that he found no signs of serious injury on the teenager's body.

Video of guards hitting and kneeing Martin Lee Anderson a day before his death drew outrage, especially in light of Dr. Charles Siebert's conclusion that the teen died of complications from a previously undiagnosed blood disorder. Siebert repeated those findings Wednesday and said that although it seemed "counterintuitive,'' the strikes were actually arousing Anderson and "probably keeping him alive.''

Siebert, who examined Anderson's body the day he died, testified in the manslaughter trial of the seven guards and a nurse of the now-closed camp.

He said the cause of death was internal hemorrhaging brought on by sickle cell trait, which can hinder cells carrying oxygen during physical stress.

Prosecutors say the guards suffocated Anderson by covering his mouth and making him inhale ammonia; a second autopsy by another doctor supports that argument. Prosecutors also say guards failed to respond to the boy's medical distress.

But defence lawyers contend that sickle cell trait was the cause of death and that their clients followed policy in trying to control a juvenile offender.

Anderson was "beyond the point of return'' when he collapsed on the exercise field while running laps, Siebert said.

Defence lawyer Robert Sombathy asked Siebert whether "knee strikes, arm-bar takedowns, pressure points, ammonia capsules or yelling in loud voices'' led to Anderson's death. He said no.

Two medical experts, including one who performed the second autopsy after Anderson's body was exhumed as part of the investigation, testified earlier in the trial that they believed the guards' actions did contribute to Anderson's death.

One doctor said Anderson died from a combination of sickle cell trait and a lack of oxygen caused by the guards. A second doctor said the guards' hands over Anderson's mouth, depriving him of oxygen, would have killed him without the underlying sickle cell trait.

Siebert's initial finding that Anderson died solely of sickle trait led to a widespread public outcry and was among the issues that prompted the governor to appoint a special prosecutor, who ordered the second autopsy.

"My career and my reputation in this state, it's already over,'' said Siebert. He testified later that the prosecutors in the trial were among those who pressured him to change his findings.

Earlier Wednesday, Guard Joseph Walsh II testified that he noticed Anderson on the boy's first day at the camp because he used profanity.

"There is no profanity at the boot camp,'' Walsh said. "They are instructed not to speak without permission to speak.''

That began a chain of events that ended when Anderson was carried off by paramedics on a stretcher, Walsh said. Anderson died at a hospital.

Walsh detailed how he used pressure points behind Anderson's ear and made him inhale ammonia to get his attention. He said he used hammer strikes on the boy's arms to get him to unclench his fists.

When Anderson's body went limp, Walsh said, he suspected Anderson of feigning illness because that tactic was common among the youths in the camp.

Walsh also said he threw the ammonia capsules he used on Anderson over the camp fence because they had the teen's saliva on them and he didn't want to put them in his pocket.

Guard Henry Dickens testified that he used his hand to hold a pressure point on the nerve behind the ear of a limp Anderson.

"If I had seen anything wrong out there I would have put an end to it,'' Dickens said. "Everything we were doing out there, we were doing things we were trained to do.''

Also Wednesday, Circuit Judge Michael Overstreet ordered the boy's father, Robert Anderson, to leave the courtroom after he supposedly made noises. He said Anderson could continue watching the trial on a video feed in another room.

The judge said Anderson and others made noises throughout the trial, which started last week. Anderson's family sat in the front of the courtroom; the noises Overstreet complained of were not heard by reporters in the back.

During the lunch break, Anderson said he did not make any noises in court and blamed the disruption on a text message he said defence lawyers sent to someone seated near him.

The boy's mother earlier left the courtroom in tears as a videotape of the guards overpowering the teen played and a defendant described how he hit the boy while he was limp on the ground.

"I cannot take it,'' Gina Jones sobbed before leaving.

The guards and nurse face as many as 30 years in prison if convicted of manslaughter.