Texas shooting: Gunman linked himself to Islamic State in a tweet

Two gunmen, who were killed while trying to attack a cartoon contest on Prophet Mohammed in Texas, were roommates, one of whom had linked himself to Islamic State fighters in a tweet posted just before the attack.

The suspects in Sunday's attack have been identified as Elton Simpson and Nadir Soofi, his roommate in an apartment in Phoenix, Arizona, CNN reported citing federal law enforcement sources.

While Simpson was convicted of making a false statement involving international and domestic terrorism in 2011, Soofi was not on the FBI's radar, officials said.

Investigators were combing through evidence retrieved from the shooters' Arizona home to help piece together a timeline of how their plot came together, the official said.

Authorities are still trying to determine the suspects' motives, US Secretary of Homeland Security Jeh Johnson said Monday.

At this point, he said, one thing appears clear: A quick-thinking police officer "likely saved a number of innocent lives."

Simpson and Soofi never made it inside the Curtis Culwell Centre in Garland, northeast of Dallas, where in addition to the cartoon contest, a right-wing Dutch politician who's on an Al Qaeda hit list was speaking Sunday evening.

A traffic officer working after-hours as security for the event and armed only with a service pistol killed both men, who were wearing body armour and carrying assault rifles, Garland Police Department spokesman Joe Harn told reporters Monday.

Investigators haven't revealed what they found in the suspects' apartment, but Simpson's social media footprint reveals one possible motive; he linked himself to ISIS in a tweet posted just before the attack.

"May Allah accept us as mujahideen," the tweet said, adding that Simpson and his fellow attacker had pledged loyalty to "Amirul Mu'mineen" (the leader of the faithful).

This description likely refers to ISIS leader Abu Bakr al Baghdadi, according to a CNN terrorism analyst.

After the shooting, an ISIS propagandist that Simpson had earlier asked his readers to follow tweeted, "Allahu Akbar!!!! 2 of our brothers just opened fire" at the Texas event.

"If there is no check on the freedom of your speech, then let your hearts be open to the freedom of our actions," tweeted the propagandist.

He was identified by two American groups that monitor jihadi websites as Junaid Hussain, a British ISIS fighter in Syria who goes by the name Abu Hussein al Britani.

In 2011, Simpson was sentenced to three years of probation after his conviction on the terror-related charge, court records show.

US authorities are investigating whether Sunday's shooting has any link to international terrorism. Simpson's tweet could indicate the attack was inspired by ISIS, but not necessarily orchestrated by the group, CNN said citing sources.