Outrage Prompts U.S. Probe Into Killing of Florida Teenager ORLANDO, Florida (Reuters) - Responding to an international petition, celebrity tweets, and spreading public outrage, the Justice Department opened an investigation on Monday into the shooting of a black teenager by a neighborhood watch captain who escaped arrest. More than 435,000 people, many alerted by tweets from celebrities like movie director Spike Lee and musician Wyclef Jean, signed a petition on Change.org, a social action website, calling for the arrest of the shooter, George Zimmerman. The Justice Department's Civil Rights Division and the FBI announced swiftly that they have opened an investigation into the shooting in Florida of 17-year-old Trayvon Martin, who was unarmed when he was killed. "The department will conduct a thorough and independent review of all of the evidence and take appropriate action at the conclusion of the investigation," the Justice Department said in a statement. The campaign to draw attention to the case is the third largest in Change.org's history, and surpassed a petition of about 300,000 signatures credited last year with persuading Bank of America to drop plans for a $5 debit card fee, said Megan Lubin, a Change.org spokeswoman. The victim's family lawyer, Ben Crump, said public pressure was behind an earlier promise by the Justice Department to review the case. And some Florida legislators are moving to consider a change in the law to prevent a recurrence. People all over the world, more than 400,000 people, said we demand you make an arrest. That's what is building pressure to look at it," Crump said. The shooting occurred February 26 when Zimmerman spotted Martin walking home from buying candy and iced tea at a convenience store. Zimmerman, patrolling the neighborhood in his car, called the 911 emergency number and reported what he called "a real suspicious guy." This guy looks like he's up to no good, or he's on drugs or something. It's raining and he's just walking around, looking about," Zimmerman told dispatchers, adding, "These assholes. They always get away. The dispatcher, hearing heavy breathing on the phone, asked Zimmerman: "Are you following him?" "Yeah," Zimmerman said. "Okay, we don't need you to do that," the dispatcher responded. But several neighbors subsequently called 911 to report a scuffle between Zimmerman and Martin. While some of the callers were still on the phone, cries for help followed by a gunshot can be heard in the background. "I recognized that (voice) as my baby screaming for help before his life was taken," Martin's mother, Sybrina Fulton, told Reuters. Police declined to arrest Zimmerman, and turned the case over to prosecutors where it remains under review. Police cited Florida's "Stand Your Ground" law, enacted in 2005 and now in effect in at least 16 other states. Dubbed "Shoot first (ask questions later) " by opponents, the Florida law allows a potential crime victim who is "in fear of great bodily injury" to use deadly force in public places. The landmark law expanded on legislation, known as the Castle Doctrine, that allowed use of deadly force in defense of "hearth and home." Passed under former Florida Governor Jeb Bush in 2005, it overturned a centuries-old doctrine that required the potential victim to retreat and avoid confrontation if possible, according to Ladd Everitt, spokesman for the Coalition to Stop Gun Violence, a Washington-based advocacy group. "No one could argue that Zimmerman could not have safely retreated and avoided this conflict, and I think that is the critical element here and why these laws are so dangerous," Everitt said. He (Zimmerman) does not have a duty to retreat in Florida. Crump said Zimmerman should not be protected under the Stand Your Ground law. "It's illogical, you can claim self defense after you chase and pursue somebody," he said. That's a courtroom defense. That's not something the police accept on the side of the street. Five years after Florida's Stand Your Ground law was enacted, a 2010 review by the St. Petersburg Times found that reports of justifiable homicides had tripled, and a majority of cases were excused by prosecutors or the courts. Meanwhile, the petition drive, started by a friend of Trayvon's mother, has been signed by people across the globe from Canada to Thailand, Lubin said. Celebrity tweets over the weekend made #Trayvon a trending topic on Twitter, she said. Additional celebrities tweeting and posting on Facebook about the case include singers Clay Aiken and John Legend, film director Michael Moore and actress Mia Farrow. "This is a great moment for the entire nation to become educated in these Stand Your Ground laws," Everitt said. It's unbelievably dangerous and really takes us to a situation where the rule of law is beginning to erode on our streets and vigilantism is being actively encouraged by these laws. Additional reporting by Tom Brown in Miami; Editing by David Adams, Kevin Gray and Christopher Wilson