A medical examiner testified that George Zimmerman's injuries were "insignificant" and could have been the result of a single blow as prosecutors continued to make their case that the neighborhood watch volunteer was not simply defending himself when he shot 17-year-old Trayvon Martin.

Dr. Valerie Rao, who was called by the prosecution to provide her assessment of Zimmerman's injuries based on photos, cast doubt on Zimmerman's claim his head was repeatedly bashed against the pavement as he fought with Martin in the Feb. 26, 2012 incident.

"The injuries are so minor that the word slam implies great force," Rao said in response to questioning by prosecutor John Guy. "There was not great force used here."

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Rao said Zimmerman's injuries were not only "not life threatening" but were also "very insignificant." She said the injuries to the back of Zimmerman's head were consistent with hitting the ground but the injuries on his face were from consistent with one punch or strike.

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Under questioning by defense attorney Mark O'Mara, Rao acknowledged she was appointed by state attorney Angela Corey, who is overseeing the prosecution of Zimmerman. She also conceded that while Zimmerman's injuries were consistent with one punch, it does not mean that he was not hit repeatedly.

"Could it of been four times?" O'Mara asked Rao, who replied, "That's not my opinion but it is possible."

Earlier, Zimmerman's best friend and the author of a book defending him testified that when he drove Zimmerman home from the Sanford Police Station, Zimmerman told him Martin grabbed hold of his gun as the two grappled in the gated community.

"Somehow I broke his grip on the gun when the guy grabbed between the grip and hammer," Mark Osterman, author of "Defending our Friend: The Most Hated Man in America," said, quoting Zimmerman whom he also he described as "stunned" and "detached" as he drove Zimmerman and his wife home.

"He had a stunned look on his face," Osterman said, "He was wide-eyed. A little bit detached."

Osterman also said that Zimmerman told him that he got out of his truck after seeing the 17-year-old Martin walking through two buildings, which he thought was suspicious.

A video was played before the court in the afternoon of Zimmerman's interview on Fox News Channel's Hannity in which he retells the events of the incident as well as his injuries.

Earlier Tuesday, the lead police detective who investigated the shooting, took the stand for a second day and said the defendant's story remained consistent throughout the investigation. Sanford Police Det. Christopher Serino, who initially wanted to charge Zimmerman with manslaughter but was overruled, said under cross examination by Zimmerman defense attorney Mark O'Mara that the neighborhood watch volunteer stuck to the same story and was cooperative. Although he said he considered Zimmerman's injuries to be minor, he said he found nothing to prompt him to doubt the 29-year-old's story, that he shot Martin in self-defense as he was being beaten.

Prosecutor Bernie de la Rionda emphasized profane language Zimmerman used on a 911 call as he observed Martin walking through the gated community, and asked Serino if Zimmerman's use of "a--holes" and "f---ing punks" indicated Zimmerman might have been profiling and following Martin.

The prosecutor also attempted to point out inconsistencies in Zimmerman's statements to Serino, including the defendant's claim that that he could not find the street address. De La Rionda also asked the former lead investigator, "Did George Zimmerman exaggerate the manner in which he was hit?" Serino replied that he felt that he did.

But O'Mara attempted to quell the prosecution's claim that Zimmerman's use of bad language show "ill will" and "spite" bay asking Serino if he would, "agree that a------ is used as a slang term?" the witness replied, "I have used it before."

O'Mara also asked Serino, "Did you think there was anything wrong with Mr. Zimmerman following Trayvon Martin?"

Serino replied, "Legally speaking, no."

Although Serino acknowledged to De la Rionda that Zimmerman had no reason to believe Martin was armed, O'Mara, noting Zimmerman claims to have shot Martin while the youth was bashing his head into the pavement, asked Serino if Martin was "armed with concrete." Serino replied that he was.

The defense attorney asked if Zimmerman's neighborhood had a "rash" of burglaries before the shooting. Serino said he did not believe there to be a crime spree but added that the defendant had believed there was.

Serino testified Monday that at first he did not believe Zimmerman's story, but when he told Zimmerman the fatal, February, 2012 encounter may have been caught on video, Zimmerman's response of "Thank God" changed his mind, Serino said.

Serino's Monday testimony became the latest prosecution witness to appear to do more harm than good to the state's case, recalling a somber conversation that touched on Zimmerman's faith and the fear he said he felt when Martin told him "You're going to die tonight."

On Tuesday, De La Rionda sought to mitigate the damage done by Serino's Monday testimony, getting the detective to concede that, as a neighborhood watch volunteer, Zimmerman would know there were no surveillance cameras. And, he said, Zimmerman would have known if Martin had taped the encounter on his cellphone.

"So he basically knew you were bluffing?" De La Rionda said, before an objection from the defense was sustained.

Zimmerman could get life in prison if convicted of second-degree murder. The state argued during its opening statement that Zimmerman profiled and followed Martin in his truck and called a police dispatch number before he and the teen got into a fight.

Over the first five days of testimony, jurors heard 911 calls from neighbors that included cries for help and the fatal gunshot. Zimmerman's attorneys are adamant that he is the one screaming on the recordings, while Martin's parents have said it's their son.

Jurors also listened to more than six hours of testimony from Martin's friend Rachel Jeantel, who testified that she was talking on the phone with the teen as the fight started.

She testified that Martin told her he was being followed by "a creepy-ass cracker." But it was her testy cross-examination exchanges with defense attorney Don West that commanded the most attention.

West attacked inconsistencies in multiple statements she gave attorneys and law enforcement officials about what she heard, including whether she heard Martin say "Get off! Get off!"

There also was conflicting testimony from neighbors that witnessed parts of the struggle between Martin and Zimmerman. Some said it appeared the larger Zimmerman was straddling Martin, but neighbor Jonathan Good said it appeared Martin was on top.

The Associated Press contributed to this report.