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 Twitter hashtags used by the account was "#texasattack."
Federal authorities confirm the account belonged to 31-year-old Elton Simpson of Phoenix who, with another gunman, opened fire last Sunday in the Dallas suburb of Garland, said Congressman Michael McCaul, chairman of the House Homeland Security Committee.
McCaul said the Twitter account linked to Simpson included images of Anwar al-Awlaki, an American-born radical cleric killed in a CIA drone strike in Yemen. But the Texas congressman stopped short of saying law enforcement had missed a red flag.
"Was he on the radar? Sure he was," McCaul said from Turkey, where he was leading a congressional delegation. "The FBI has got a pretty good programme to monitor public social media."
The Department of Homeland Security (DHS) and the FBI issued a joint intelligence bulletin to local law enforcement on April 20 warning that the Garland event was a possible target for a terrorist attack, according to a DHS official.
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.
And a US law enforcement official said authorities had an open investigation into Simpson at the time of the shooting. The official was not authorised to discuss ongoing investigation by name and spoke on condition of anonymity.
It's unclear why Simpson and his roommate, Nadir Soofi, were not stopped. A security guard was wounded in the leg before the gunmen were killed at the scene.
The law enforcement official said investigators will be studying the contacts the men had prior to the shooting, both with associates in the US and abroad, to determine any additional terror-related ties.