PHOENIX: About 20 minutes before the shooting at a Texas cartoon contest that featured caricatures of Prophet Muhammad (PBUH), 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 hashtags 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 authorised to be quoted discussing the document.

Social media accounts linked to “violent extremists” had been focusing on the contest, the bulletin said.

A federal 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.

McCaul said the Twitter account linked to Simpson included images of Anwar al-Awlaki, a US-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 program to monitor public social media."

The self-styled Islamic State (IS) group claimed responsibility Tuesday for the shooting, but counter-terrorism experts said IS has a history of asserting involvement in attacks in which it had no operational role.

That suggests the two gunmen could have carried out their own lone wolf-style strike.

The evidence does not indicate the attack was directed by the Islamic State group, “but rather inspired by them,” McCaul said. “This is the textbook case of what we're most concerned about."

The postings on the Twitter account linked to Simpson contrast sharply with the impression the jovial man and his quiet, 34-year-old roommate gave to neighbours and the leader of the mosque, which they attended in Phoenix up until recently.

The families of both men say they were shocked by what happened and never saw any signs that either of them was capable of such violence. Both men had had run-ins with the law, according to court records.

Simpson, who was born in Illinois, was arrested in 2010 after being the focus of a four-year terror investigation.

But despite amassing more than 1,500 hours of recorded conversations, including Simpson's discussions about "fighting nonbelievers for Allah" and plans to link up with “brothers” in Somalia, the government prosecuted him on only one minor charge — lying to a federal agent.

He was sentenced to three years of probation and ordered to pay $600 in fines and court fees.

It's unclear at what point in his life Simpson turned radical.

Simpson played basketball as a freshman at Yavapai College, a junior college in Prescott, Arizona, for the 2002-2003 season before leaving school, said then-assistant coach Jeff Renegar.

A former teammate, Keion Kindred, said the two would discuss everything from family life, movies and cartoons, to their love for basketball and their ability to play pool.

“Elton was a good kid, he was a comedian of some sort. We were young, so he was like every 18- to 19-year-old in college, trying to have fun and figure it out,” said Kindred, who lost contact with him after 2005.

It's not known how or when Simpson met Soofi.

Soofi was born in the Dallas area, raised Muslim and later spent part of his childhood in Pakistan, according to his family.

Soofi was an undergraduate pre-medicine major at the University of Utah from fall of 1998 to the summer of 2003, said university spokeswoman Maria O'Mara. She said he did not earn a degree.

Utah court records show Soofi had several brushes with police during his time in the state.

He pleaded to possession of alcohol by a minor, alcohol-related reckless driving and driving on a suspended license in 2001, court records show, and misdemeanor assault the following year.

Simpson had worshipped at the Islamic Community Center of Phoenix for about a decade, but he quit showing up over the past two or three months, the president of the mosque told The Associated Press.

The centre's president, Usama Shami, said Simpson would play basketball with mosque members and was involved with the community.

Soofi owned a nearby pizza business and would stop in to pray occasionally, sometimes bringing with him his young son, he said.

“They didn't show any signs of radicalisation,” Shami said.

IS recently urged those in the United States, Europe and Australia who cannot safely travel to fight in Syria and Iraq to carry out 'jihad' in the countries where they live.

An audio statement on the extremist group's Al Bayan radio station called the men “two soldiers of the caliphate".