The United States Justice Department opened an investigation into the murder of 17-year-old Trayvon Martin on Monday night, just as online outcry for action reached a fever pitch.

Mentions of Martin's death nearly quadrupled on Twitter last night, where retweets from celebrities like director Spike Lee, football star Chad Ochocinco and media tycoon Russell Simmons helped turn the names of Martin, a Sanford, Fla., resident and accused killer George Zimmerman into trending topics.

Martin was fatally shot while returning from a convenient store on Feb. 26. Zimmerman, the neighborhood watch captain responsible, was not arrested or given a sobreity test after the incident. 

According to Topsy, Martin's name has been tweeted over 73,000 times in the past 24 hours, and Zimmerman's has received 21,000 mentions. Over 7,000 of those tweets included a link to an article by globalgrind.com’s Michael Skolnik entitled "White People, You Will Never Look Suspicious Like Trayvon Martin," which received more than 57,000 likes on Facebook.

The conversation also helped propel a petition started by Martin's family to arrest George Zimmerman register over 520,000 signatures, the third-most signatures a change.org petition has ever received, according to a report from msnbc.com.

Speaking after the Justice Department's announcement became public, Martin family attorney Ben Crump credited the groundswell of attention online for forcing the investigation.

"People all over the world, more than 400,000 people, said we demand you make an arrest," Crump said. "That's what is building pressure to look at [the case]."

At 10:20 am Eastern Time, Florida state attorney Norm Wolfinger announced that Seminole County would open an investigation into the murder, saying a county grand jury would receive evidence on April 10. 

Photo via change.org