However, Serino also said that he didn't feel there was "active deception" on Zimmerman's part when he said he didn't know where he was and had to get out of his vehicle to look at a street sign.
About the fight that allegedly ensued between Zimmerman and Martin, Serino also said Zimmerman's nose may have been bleeding back into his mouth, which could explain why Martin didn't have blood on his hands. And he agreed with O'Mara that the purported smothering of Zimmerman by Martin could have happened only momentarily, perhaps not long enough to be heard on the 911 call made by a neighbor.
When questioned again by the prosecution, Serino admitted that he was speculating on the details of how the fight played out.
The last witness to take the stand before court recessed for lunch was Mark Osterman, who called Zimmerman "the best friend I've ever had."
Osterman has worked in law enforcement for more than 20 years and said he's the one who helped Zimmerman purchase his gun. "He asked whether he should or shouldn't -- to start with -- and I recommended that he should. Anybody who's a non-convicted felon should carry a firearm. The police aren't always there," said Osterman.
Osterman also recounted the story of the shooting that Zimmerman told him. He said that as the two scuffled, Zimmerman's jacket came up, potentially exposing his gun to Martin. Osterman said Zimmerman was mostly focused on Martin's hands, which he said were keeping him from breathing.
"It was critical. He was losing oxygen. He felt he was not able to breathe. That's why he was desperate to clear an airway," said Osterman.
Osterman said Zimmerman felt Martin grab either his gun or the holster.
"That's when he had to -- he freed one of his hands and got the gun. He either broke contact or knocked Trayvon's hand away, and then he drew it," said Osterman.
Osterman said Zimmerman shot Martin, crawled out from under him, holstered his weapon, got on Martin's back and held his arms out, pinning them down.
But a photograph snapped by a neighbor shows Martin's hands under his body. Osterman said he wasn't aware of this fact.
Martin died of a single gunshot wound to the chest.
Tuesday afternoon, prosecutors called Dr. Valerie Rao, chief medical examiner for Duval County, Florida, to the stand to discuss the injuries Zimmerman allegedly sustained the night of the shooting.
Rao said the injuries to the back of Zimmerman's head were not consistent with being repeatedly slammed onto concrete. She said his head may have hit concrete once or maybe twice.
The doctor also said Zimmerman's facial injuries could have been created with one blow to the face, an assessment that seemingly contradicts what Zimmerman said in an interview with Fox's Sean Hannity that aired on July 18, 2012. The interview was played for the jury before Rao took the stand. Zimmerman told Hannity that Martin may have hit him the face a dozen times.
Rao said Zimmerman's injuries were "small" and "insignificant." He did not require any sutures, and he only seemed to need Band-Aids, according to Rao.
After Rao finished her testimony, prosecutors called Sanford police latent print analyst Kristen Bentsen to the stand. Bentsen said no useable fingerprint or palm print was recovered from Zimmerman's firearm. This evidence could contradict Zimmerman's claim that Martin grabbed his gun.
O'Mara asked Bentsen if environmental factors like rain can damage latent prints, and she replied "yes." It was raining periodically the night of the shooting.
Court ended early Tuesday to give attorneys time to prepare their arguments for a hearing regarding the admissibility of evidence of Zimmerman's interest in the criminal justice field, including his course work pursuing a criminal justice degree, his rejected application to become a police officer and his request to do a ride-along with police.
Prosecutor Rich Mantei argued the evidence is relevant, because it proves Zimmerman wanted to catch bad guys, and it may show that Zimmerman knew what to say to law enforcement when they arrived at the scene.
Attorneys will present their arguments outside the presence of the jury when court resumes Wednesday morning at 8:30 a.m. ET.