Story highlights George Zimmerman is charged with murder in the death of Trayvon Martin, 17

His lawyers want to loosen restrictions on Zimmerman, saying he lives in fear

The defense asks to talk to an investigator about a pleading voice on a 911 tape

Defense says Zimmerman killed the unarmed Martin in self-defense

The defense team for George Zimmerman -- charged with murder in 17-year-old Trayvon Martin's death -- continued their offensive Friday, pressing in court for access to information they hope will shed light on what happened and exonerate the Florida man.

In an interview Thursday with "InSession's" Jean Casarez, Zimmerman's lawyer Mark O'Mara previewed the court action and reiterated the position that Martin, and not his client, was the aggressor in the February 26 incident.

"We will have a ... trial, and if that trial is fair, he's going to be acquitted," O'Mara said, ruling out a deal in which his client pleads guilty to lesser charges.

While driving through his Sanford, Florida, neighborhood that winter night, Zimmerman spotted Trayvon Martin, whom he described on a 911 call as "suspicious." A dispatcher told the 28-year-old neighborhood watch volunteer not to follow Martin, but he did anyway. After an altercation, the unarmed African-American teenager was shot dead.

Weeks after the incident, the case drew national media attention, and activists demanded an arrest. On April 11, Zimmerman was charged with second-degree murder.

Defense lawyers have steadfastly maintained their client's innocence, saying Zimmerman was badly beaten and acted in self-defense. They have challenged key pieces of the prosecution's case.

One of the most potentially pivotal pieces deals with a 911 call from that night in which someone can be heard screaming for help. The thinking is that the person pleading for assistance -- whether it was Martin or Zimmerman -- was on the defensive, and the other was the aggressor.

A defense motion filed Monday in Seminole County court, which the prosecution replied to Friday, asks that Zimmerman's lawyers be allowed to question for a second time Sanford police investigator William Erwin. Zimmerman's lawyers claim that Erwin overheard the late teen's father say the screaming voice was not that of his son. The father, Tracy Martin, has said he is "positive" the pleading voice is that of Trayvon.

In a response Friday, Assistant State's Attorney Bernardo de la Rionda criticized the defense's arguments and said Erwin "never mentioned anything about overhearing Tracy Martin talk about the voice." But the lead prosecutor didn't object to O'Mara and other lawyers deposing Erwin.

Defense lawyers say Trayvon Martin, 17, gave George Zimmerman a beating and Zimmerman acted in self-defense.

Additionally, Zimmerman's defense team filed several new documents Friday.

One asks that the prosecution "disclose all witnesses known to the state who claim to be familiar with Trayvon Martin's or George Zimmerman's voice that have heard" the 911 call.

Another motion asks the court to force Benjamin Crump, the Martin family's attorney, to turn over the original recording and recording device that he used to first interview the late teenager's girlfriend.

Asserting that the young woman can prove Martin was killed "in cold blood," Crump said in March that the girlfriend "completely blows Zimmerman's absurd self-defense claim out of the water." But defense lawyer Don West said Thursday that the recordings he's aware of are a "copy of a copy that's disjointed and chopped and obvious that it was edited."

On Friday, Crump said he has already turned over the tape. "The phone records speak for themselves," the lawyer said. "She tells him to run. He runs. And then, when you go through the 911 tape, George Zimmerman admits that he is pursuing Trayvon."

Zimmerman filed a different motion Friday, asking that he be allowed to remove his GPS monitor and leave Seminole County without the court's permission. The document alleges that Zimmerman and his wife have lived "in fear of violence at the hands of those who would wish (him) harm based on their misinterpretation of the events."

These and other court filings Friday follow other moves by Zimmerman's team in recent days to raise questions in court and in public about the prosecution's case. The next hearing in the case is set for December 11.

Last week, they released a photo of a bloodied Zimmerman taken a few minutes after Martin was shot dead. On Thursday, they filed a lawsuit against NBC for a report that O'Mara claims portrays his client as a racist and murderer; the lawyer called it "one shining example of what was done wrong to George."

"They took an event and made it highly inflammatory," said O'Mara of the NBC report that, according to the lawsuit, selectively edits audio of Zimmerman's 911 call to make him sound racist. "They wanted to be first, and ... they wanted to have the most sensational headline of that day. And they just did it. They just presumed, I think, ... they had a racist."

Zimmerman, who is Hispanic, is accused of fatally shooting Martin, who was African-American.

NBC Universal disputed the accusations and issued a statement saying: "There was no intent to portray Mr. Zimmerman unfairly. We intend to vigorously defend our position in court."