GWEN IFILL: The FBI looked for terrorist tie-ins today after two men attacked an exhibit of Muhammad cartoons outside Dallas.

The Sunday evening shooting in garland, Texas sent investigators across the southwest.

Federal agents searched this apartment in Phoenix, Arizona — within hours of the shooting — along with a white van and a second vehicle.

DOUGLAS HAYES, Neighbor of gunmen: They were out here just wailing on the back door over here trying to get it open. There were several different guys that came over and were trying to cut into the door.

GWEN IFILL: The FBI said the apartment belonged to Elton Simpson. He was identified as an American Muslim convicted in 2011 of lying to the FBI in a terror investigation.

Investigators say he sent out several tweets before Sunday's shooting, using the hashtag “Texas attack.”

Various news accounts identified the second man as Nadir Soofi, Simpson's roommate.

Clad in body armor, the two gunmen fired assault-style rifles and wounded a security guard at the garland event center.

An off-duty traffic officer, also working security, returned fire, killing both attackers.

JOE HARN, Garland Police Department Spokesman: He did what he was trained to do and, under the fire that he was put under, he did a very good job and probably saved lives. We think their strategy was to get to the event center, into the event center, and they weren't able to get passed that outer perimeter that we had set up.

GWEN IFILL: The bodies were still there this morning, covered by a tarp, while a bomb squad searched the gunmen's car, and blew up several suspicious objects but found no bombs.

Police said they're still working on the motive, but the event center was hosting a contest for cartoons depicting the prophet Muhammad. That's considered blasphemous under Islamic tradition.

It was organized by a group led by New York activist Pamela Geller, who campaigns against what she calls “Islamic supremacism”.

Because of that, garland police had created a security plan months in advance.

They drew on what happened in Paris last January when Islamism gunmen killed a dozen people at the office of a French magazine after it published Muhammad cartoons.