Serino on Monday said he thought Zimmerman’s injuries were “lacking” when compared to Zimmerman’s statement that Trayvon punched him 25 to 30 times and repeatedly slammed his head against a concrete walkway until Zimmerman said he “felt like it was going to explode.”

“I still kept an open mind that he could be a victim,” Serino testified.

Serino has called the events that unfolded “avoidable” and had recommended manslaughter charges against Zimmerman. The 15-year veteran later told FBI investigators that he was pressured by black officers into filing charges and did not believe there was enough evidence to support charges. That shift in stance could raise questions about the credibility of the law-enforcement witness in the murder trial.

Serino hired lawyer Jose Baez, well-known after his successful representation of Casey Anthony in her 2011 murder trial. Baez, who no longer represents Serino, told NBC’s Today show on Monday that Serino “did exactly what he was told through the course of the investigation, until it became outright political.”

A large portion of Monday’s hearing consisted of jurors hearing recordings and watching video of Zimmerman’s statements to police.

In the first one, less than two hours after the shooting, Zimmerman told Singleton he “screamed ‘Help me!’ maybe 50 times” after Trayvon ambushed him in the dark, beating the neighborhood watch volunteer and telling him, “You’re going to die tonight.”

Zimmerman went on to tell Singleton that Trayvon mounted him on the sidewalk, grabbed his head and “wailed it” into the concrete. Zimmerman said he slid onto grass to try to get out from underneath Trayvon.

“I’m still yelling for help. ‘Help me, help me, he’s killing me,’” Zimmerman said during the interview in the police station. “He says, ‘You’re going to die tonight.’ ...

“I felt his hand go down my side, and I thought he was going for my firearm, so I grabbed it immediately, and as he banged my head again, I just pulled out my firearm and shot him.”

Zimmerman reported hearing Trayvon say, “All right, you got it, you got it,” after Zimmerman shot him. Zimmerman said he got on top of Trayvon and held down his hands -- “because he was still talking” -- causing Trayvon to say, “Ow, ow.”

In the initial police interview, Zimmerman told Singleton he was suspicious of Trayvon because the teen was walking casually in the rain and there had been several recent break-ins in the Retreat at Twin Lakes neighborhood.

“These guys always get away,” Zimmerman said to Singleton, acknowledging he had previously reported suspicious people to police but had never before seen Trayvon. “It’s always dark. They always come around at nighttime.”

After the audio recording of Zimmerman’s statement was played in court, prosecutor Bernie de la Rionda asked Singleton to read a written statement Zimmerman gave to her the night of the shooting. Zimmerman referred to Trayvon as “the suspect” in the written statement and added details that he did not mention in the verbal interview.

Also on Monday, an FBI voice-recognition expert testified that it was scientifically impossible to determine whose voice is screaming for help in a 911 recording of the fight and shooting.

Hirotaka Nakasone said he was able to isolate less than 3 seconds of uninterrupted screams from the 911 call, which he said was not enough for accurate voice comparison.

He did say, however, that people familiar with the voices in question may be better able to identify the voices. Trayvon’s parents, who say the screams belong to their son, and Zimmerman’s father, who thinks the screams are his son’s, are both expected to testify at some point.

Miami Herald staff writer David Ovalle contributed to this report.