A prosecutor in George Zimmerman's murder trial today tried to pick apart the statements of a Sanford, Fla., police detective who had been called as a prosecution witness a day before but gave testimony that seemed to benefit the defence.

Prosecutor Bernie de la Rionda asked the judge to strike from the record a statement Detective Chris Serino made Monday in which he said he found credible Zimmerman's account of how he got into a fight with Trayvon Martin.

Zimmerman has pleaded not guilty to second-degree murder in 17-year-old's fatal shooting last year, arguing he acted in self-defense.

De la Rionda argued the statement was improper because one witness isn't allowed to give an opinion on the credibility of another witness. Defence attorney Mark O'Mara argued it was proper because it was Serino's job to decide whether Zimmerman was telling the truth.

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George Zimmerman faces a charge of second-degree murder in the death of teenager Trayvon Martin as his trial in Sanford, Fla., is in its second week. (Orlando Sentinel, Joe Burbank, Pool/Associated Press)

Judge Debra Nelson told jurors to disregard the statement.

"This is an improper comment," the judge said.

The prosecutor then questioned Serino about his opinion that Zimmerman didn't display any ill will or spite to Martin. Prosecutors must prove there was ill will, spite or a depraved mind by the defendant to get a second-degree murder conviction.

The prosecutor played back Zimmerman's call to police to report the teen wailing through his gated community. Zimmerman uses an expletive, refers to "punks" and then says, "These a———-. They always get away."

The detective conceded that Zimmerman's choice of words could be interpreted as being spiteful.

The prosecutor also challenged Serino's contention that Zimmerman's story didn't have any major inconsistencies.

The prosecutor played back Zimmerman's police interview and noted that investigators were asking about small differences in the neighbourhood watch volunteer's story. Zimmerman claimed he spread out the teen's arms after the shooting. But a photo taken immediately afterward shows Martin's arms under his body.

"Is that inconsistent with the defendant's statement he spread the arms out?" de la Rionda asked.

"That position, yes it is," Serino said, though he later noted that Zimmerman's description was consistent with the medical examiner's report.

Later, in response to Serino's cross-examination testimony that he'd seen a convenience store video that showed Martin in a hooded sweatshirt, de la Rionda asked, "Are you saying in Seminole County, it's illegal for someone to wear a hoodie at night?"

"No sir. I'm not," Serino said.

Zimmerman has said he fatally shot the unarmed black teen in self-defence in February, 2012, because he says Martin was banging his head into a concrete sidewalk behind townhomes in a gated community. Zimmerman, 29, could get life in prison if convicted of second-degree murder.

The state argued during its opening statement that Zimmerman profiled Martin from his truck and called a police dispatch number before he and the teenager got into a fight. Zimmerman has denied the confrontation had anything to do with race, as Martin's family and their supporters have claimed. A 44-day delay in Zimmerman's arrest led to protests around the nation; he was ultimately charged by a Florida special prosecutor. Zimmerman's father is white and his mother is Hispanic.

Prosecutors on Tuesday also asked the judge to allow them to introduce school records showing Zimmerman took a class that addressed Florida's self-defense law. They say it will show he had knowledge of the law, even though he claimed he didn't in an interview with talk show host Sean Hannity. The interview was played for jurors.

O'Mara objected, saying the records were irrelevant. He referred to the prosecution's efforts to introduce them as "a witch hunt."

Prosecutors also want to introduce a job application Zimmerman made to a police agency in Virginia and an application to ride around with Sanford police officers.

The judge said she would rule later Tuesday.

Late in the morning the prosecution called Mark Osterman, a federal air marshal who described Zimmerman as "the best friend I've ever had."

He testified that he spoke with Zimmerman both the night of and the day after the shooting. Osterman later wrote a book about his recollections of what Zimmerman told him.

Under questioning by de la Rionda, Osterman said that Zimmerman told him Martin had grabbed his gun during their struggle, but that Zimmerman was able to pull it away.

That account is different from what Zimmerman told investigators in multiple interviews when he only said it appeared Martin was reaching for his gun prior to the shooting. He never told police the teen grabbed it.

"I thought he had said he grabbed the gun," Osterman said. "I believe he said he grabbed the gun."

Prosecutors also called to the witness stand a medical examiner from Jacksonville who didn't perform the autopsy on Martin but was asked by prosecutors to review evidence in the case. Dr. Valerie Rao testified that Zimmerman's injuries were insignificant, bolstering the prosecution's claims that Zimmerman's life wasn't in jeopardy during his fight with Martin.

"They were so minor that the individual who treated and examined Mr. Zimmerman decided stitches weren't required," Rao said.

Inquiry requested into Instagram photo

In other news related to the trial, prosecutors have asked the judge to conduct an inquiry into an Instagram photo that was posted last week by the daughter of defence attorney Don West.

The photo shows West with his daughter, Molly, holding up ice cream cones. The caption reads: "We beat stupidity celebration cones" and "(hash)dadkilledit."

The prosecution motion said the photo was posted after West's often tense cross-examination with prosecution witness Rachel Jeantel, who was on the phone with Martin in the moments before he was fatally shot by Zimmerman during a fight.

Prosecutors said an inquiry is necessary "to ensure that witnesses and court proceedings are treated with respect and not as occasions for inappropriate jokes."

A spokesman for Zimmerman's defence team didn't immediately respond to an email inquiry about the motion. But spokesman Shawn Vincent released a statement last weekend saying West's daughter wasn't responding to a specific witness.

"The defence team is highly sensitive to the issues that surround this case, and we understand that the way we conduct ourselves during trial is at least as important as the verdict," the statement said. "And Molly's comments on Instagram are contrary to everything we stand for."