March 19, 2012 -- The FBI, the Justice Department's Civil Rights Division, and the U.S. Attorney's Office for the Middle District of Florida will investigate the killing of Trayvon Martin, an unarmed 17-year-old Florida high school student, who was shot by a self-appointed neighborhood watchman.

"The department will conduct a thorough and independent review of all of the evidence and take appropriate action at the conclusion of the investigation," a statement released by the Justice Department this evening said. "The department also is providing assistance to and cooperating with the state officials in their investigation into the incident."

George Zimmerman, who claimed self-defense in shooting Martin, has not been charged with any crime in the fatal shooting of the teenager on Feb. 26.

Outrage over the incident has only grown, as irregularities in the police investigation have been uncovered and 911 tapes released last week showed Zimmerman ignored a dispatcher's directive not to pursue the teen.

Zimmerman also blatantly violated major principles of the Neighborhood Watch manual, ABC News has learned.

The manual, from the National Neighborhood Watch Program, states: "It should be emphasized to members that they do not possess police powers, and they shall not carry weapons or pursue vehicles. They should also be cautioned to alert police or deputies when encountering strange activity. Members should never confront suspicious persons who could be armed and dangerous."

Yet Zimmerman made a nonemergency call to police and pursued him anyway, even though 911 dispatcher told him not to, before fatally shooting the teenager.

The Sanford, Fla., Police Department, relenting to massive public pressure, released parts of these 911 tapes pertaining to the shooting.

On the tapes, Zimmerman, claiming Martin looked intoxicated, says he saw the teenager cut through from the main street in the tidy Retreat at Twin Lakes, onto a path between two blocks of townhouses, and decided to gave chase. Zimmerman told a police dispatcher, "These a**holes, they always get away"

Dispatcher: "Are you following him?"

Zimmerman: "Yes."

Dispatcher: "OK, we don't need you to do that."

Zimmerman didn't stop and ultimately confronted Martin, who was carrying a package of Skittles and a bottle of iced tea. The two allegedly scuffled. In a series of seven 911 calls, eyewitnesses reported that they heard a sort of howling. One caller said, "they're wrestling right in the back of my porch, one man is yelling help."

Then a gun shot rang out. The howling and yelling stopped.

Zimmerman wasn't arrested at the time, and ABC News has learned he was not given a drug or alcohol test that night -- standard in most homicide investigations.

Law enforcement expert Rod Wheeler, who listened to the tapes, told ABC News that Zimmerman, not Martin, sounded intoxicated in the police recordings of the 911 calls.

"When I listened to the 911 tape, the first thing that came to my mind is this guy sounds intoxicated. Notice how he's slurring his words. We as trained law enforcement officers, we know how to listen for that right away, and I think that's going to be an important element of this entire investigation," Wheeler said.

According to Chris Tutko, the director of the National Neighborhood Watch Program, there are about 22,000 registered watch groups nationwide, and Zimmerman was not part of a registered group -- another fact the police were not aware of at the time of the incident.

ABC News' Bradley Blackburn and Seni Tienabeso contributed to this report.