U.S. officials are trying to learn if there are any links between the Islamic State group and two gunmen who attacked a gathering in Garland, Texas Sunday. A security guard was injured. Police killed both gunmen.

Investigators looked at a Twitter account linked to one of the men as well as electronic devices used by both. On Wednesday, officials said a message was posted on the Twitter account about 20 minutes before the shooting. The tweet said: "May Allah accept us as mujahideen," or holy Muslim fighters.

On Tuesday, the Islamic State group claimed responsibility for the attack. Police identified the gunmen in the Garland attack as Elton Simpson, 31, and Nadir Soofi, 34.