PHOENIX — About 20 minutes before the shooting at a Texas cartoon contest that featured images of the Prophet Muhammad, a final tweet posted on an account linked to one of the gunmen said: “May Allah accept us as mujahideen,” or holy warriors.

Among the hastags used by the account was “(hash)texasattack.”

Federal authorities were tracking the Twitter account linked to 31-year-old Elton Simpson of Phoenix before he and another gunman opened fire Sunday in the Dallas suburb of Garland, said Rep. Michael McCaul, chairman of the House Homeland Security Committee, who was briefed on the investigation by federal law enforcement officials.

The Department of Homeland Security and the FBI on April 20 also had issued a joint intelligence bulletin to local law enforcement warning that the Garland event was a possible target for a terrorist attack, according to a DHS official who was not authorized to be quoted discussing the document.

Social media accounts linked to “violent extremists” had been focusing on the contest, the bulletin said. According to mainstream Islamic tradition, any physical depiction of the Prophet Muhammad — even a respectful one — is considered blasphemous.