Sunday's shooting attack on a Muhammad cartoon exhibit and contest in Garland, Texas appears to be a largely spontaneous reaction to an "unprecedented avalanche" of vicious social media posts sent only a few days before the event from jihadists around the world urging massive violence including the murders of the events' organizers and all others in attendance, a law enforcement source told the Investigative Project on Terrorism.

The gunmen, identified as Phoenix, Ariz. residents Elton Simpson and Nadir Soofi, drove up to the Curtis Culwell Center as a Muhammad Art Exhibit and Contest was wrapping up Sunday evening. They started firing automatic weapons as soon as the left Soofi's car, wounding an unarmed security officer.

A Garland police traffic officer who was with the injured man returned fire, killing Simpson and Soofi, police say.

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