ORLANDO, Fla., March 13, 2012 -- ABC News has uncovered questionable police conduct in the investigation of the fatal shooting of an unarmed black teenager by a white neighborhood watch captain in Florida, including the alleged "correction" of at least one eyewitness' account.

Sanford Police Chief Billy Lee said there is no evidence to dispute self-appointed neighborhood watch captain George Zimmerman's assertion that he shot 17-year-old Trayvon Martin out of self-defense.

"Until we can establish probable cause to dispute that, we don't have the grounds to arrest him," Lee said.

Martin had been staying at his father's girlfriend's house during the night of the NBA All-Star game Feb. 26.

The teenager went out to get some Skittles and a can of ice tea. On his way back into the gated suburban Orlando community, Martin, wearing a hood, was spotted by Zimmerman, 28.

According to law enforcement sources who heard Zimmerman's call to a non-emergency police number, he told a dispatcher "these a..holes always get away."

Zimmerman described Martin as suspicious because he was wearing a hooded sweatshirt and walking slowly in the rain, police later told residents at a town hall.

A dispatcher told him to wait for a police cruiser, and not leave his vehicle.

But about a minute later, Zimmerman left his car wearing a red sweatshirt and pursued Martin on foot between two rows of townhouses, about 70 yards from where the teen was going.

Lee said Zimmerman's pursuit of Martin did not of itself constitute a crime.

Witnesses told ABC News a fist fight broke out and at one point Zimmerman, who outweighed Martin by more than 100 pounds, was on the ground and that Martin was on top.

Austin Brown, 13, was walking his dog during the time of the altercation and saw both men on the ground but separated.

Brown along with several other residents heard someone cry for help, just before hearing a gunshot. Police arrived 60 seconds later and the teen was quickly pronounced dead.

According to the police report, Zimmerman, who was armed with a handgun, was found bleeding from the nose and the back of the head, standing over Martin, who was unresponsive after being shot.

An officer at the scene overheard Zimmerman saying, "I was yelling for someone to help me but no one would help me," the report said.

Witnesses told ABC News they heard Zimmerman pronounce aloud to the breathless residents watching the violence unfold "it was self-defense," and place the gun on the ground.

But after the shooting, a source inside the police department told ABC News that a narcotics detective and not a homicide detective first approached Zimmerman. The detective pepppered Zimmerman with questions, the source said, rather than allow Zimmerman to tell his story. Questions can lead a witness, the source said.

Another officer corrected a witness after she told him that she heard the teen cry for help.

The officer told the witness, a long-time teacher, it was Zimmerman who cried for help, said the witness. ABC News has spoken to the teacher and she confirmed that the officer corrected her when she said she heard the teenager shout for help.

The Sanford Police Department refused to release 911 calls by witnesses and neighbors.

Several of the calls, ABC News has learned, contain the sound of the single gunshot.

Lee publically admitted that officers accepted Zimmerman's word at the scene that he had no police record.

Two days later during a meeting with Trayvon's father Tracy Martin, an officer told the father that Zimmerman's record was "squeaky clean."

Yet public records showed that Zimmerman was charged with battery against on officer and resisting arrest in 2005, a charge which was later expunged.

Zimmerman has not responded to requests for a comment.

"I asked [the police] well did you check out my son's record?" Tracy Martin told ABC News in an interview Sunday. "What about his?...Trayvon was innocent."

Trayvon Martin had no arrest record or disciplinary action for violence as a student in North Miami's Krop High School.

On Monday Lee, seeking to head off racial unrest, tried to reassure the public that his department was doing all it could to reach a fair conclusion, as some in the crowd heckled him saying "a little black boy is dead."

Lee's department said it plans on passing its investigation over to the state's attorney office to determine whether or not to press charges against Zimmerman.

Trayvon Martin's parents described him as the kind of son who even at 17, allowed his parents to kiss him publicly.

"That was my baby, my youngest son," mother Sybrina Fulton told ABC News in an interview in Miami. "He meant a lot to me, I don't think the police department really understands that…I need justice for my family, I just want justice for my son."

Fulton is incensed that Zimmerman left his car despite being urged against it by dispatchers to stay put.

"My son didn't do anything he was walking home from the store. Why would the neighborhood watch guy would have a weapon?...It's just crazy.You are supposed to watch the neighborhood, not take the law into your own hands."

ABC News' Matthew Rosenbaum contributed to this report.