SANFORD, Fla. — Prosecutors are winding down their case in George Zimmerman’s murder trial after presenting forensic evidence and testimony aimed at refuting Zimmerman’s claim he was acting in self-defense when he fatally shot Trayvon Martin.

It is still to be seen if they will call one or both of Martin’s parents to the witness stand to testify about whose voice is on 911 recordings of a fight between Zimmerman and Martin that preceded the shooting.

Martin’s parents, Sybrina Fulton and Tracy Martin, contend it is their 17-year-old son screaming on the calls, while Zimmerman’s father has said it is his son. The screams are crucial pieces of evidence because they could determine who the aggressor was in the confrontation.

Testimony resumes and prosecutors are expected to rest their case Friday, after jurors return from the Fourth of July holiday.

On Wednesday, prosecutors called forensics experts and college professors who they hoped would weaken Zimmerman’s claims of self-defense and professed ignorance of Florida’s “stand-your-ground” law.

Florida Department of Law Enforcement DNA expert Anthony Gorgone on Wednesday testified that Martin’s DNA was not found on the grip of George Zimmerman’s gun, and Zimmerman’s DNA was not found under the unarmed teen’s fingernails. Zimmerman said he shot Martin in the chest to protect himself as Martin reached for his gun during a fight.

While cross-examining Gorgone, defense attorney Don West focused on the packaging of the DNA samples, suggesting they could have led to the samples being degraded. Gorgone told him Martin’s two sweatshirts had been packaged in plastic while wet, instead of a paper bag where they can dry out, and when he opened the samples they smelled of ammonia and mold.

Florida Department of Law Enforcement analyst Amy Siewert also testified that tearing and residue on Martin’s clothing showed the gun was against him when it fired.

The forensic experts’ testimony came on the same day prosecutors presented evidence about Zimmerman’s work in a college criminal justice course, which they said shows the former neighborhood watch volunteer knew about Florida’s self-defense law and had aspirations of becoming a police officer. They also showed he had applied for a police officer job in northern Virginia and applied to go on ride-alongs with Sanford police officers.

Prosecutors have sought to portray Zimmerman as a vigilante who profiled Martin as the teen walked through the central Florida gated community on a rainy night.

Zimmerman had maintained in an interview with Fox News last year that he did not know about the law. Prosecutors said he did have knowledge of it, however, because the subject was covered in the college class. They called as a witness Alexis Francisco Carter, the military attorney who taught Zimmerman’s class that covered the “stand-your-ground” law, which states a person has no duty to retreat and can invoke self-defense in killing someone if it is necessary to prevent death or great bodily harm. Carter described Zimmerman as one of his better students and said he got an “A” in his class.

Prosecutors said Zimmerman’s ability to understand criminal investigations and desire to be a police officer doesn’t show wrongdoing but is relevant to Zimmerman’s state of mind on the night Martin was killed.

“He has applied to be a police officer before; he still wants to be one, according to some of his homework assignments. ... This wasn’t some sort of passive thing,” said prosecutor Richard Mantei.