​FBI issued warning ahead of Texas attack

WASHINGTON - FBI Director James Comey said Thursday his agency had looked into Elton Simpson, one of the gunmen involved in the attack on a prophet Muhammad art event in Garland, Texas on Sunday.

Just before the attack Simpson tweeted his allegiance to ISIS with the hashtag #TexasAttack. That same day -- at about 4:00 p.m. -- the FBI says it sent a bulletin with photographs of Simpson, 31, and a license plate to local police in Garland.

But a Garland Police Department spokesperson told CBS News it had "no information" that Simpson and accomplice Nadir Soofi were headed their way. The FBI believed Simpson might be interested in the controversial event but did not know Simpson and Soofi were en route from Phoenix, armed with AK-47s.

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Elton Simpson and Nadir Soofi, the two gunmen killed while trying to attack a Muhammad art event in Garland, Texas CBS News

At around 7:00 p.m. on Sunday the two men opened fire on police before being killed by a traffic officer. Simpson had apparently received guidance and inspiration online from American jihadist Mushahid Miski.

The FBI had reopened an investigation into Simpson in March, but he was not being watched around the clock.

Thursday, in an off camera session with reporters, FBI Director James Comey described the "siren song" of ISIS. He said the terror group reaches out through social media sites like Twitter, identifies followers, and sends them to encrypted websites that are difficult to track.

Comey said the group's message is "go kill people." He added that the group sometimes supplies hit lists and sometimes suggests particular targets.