Islamic State Claims US Prophet Cartoon Attack

The Islamic State group has claimed responsibility for Sunday's attack on an event in Texas where cartoons of the Prophet Mohammed were on display.

An audio statement on the extremist group's Al Bayan radio station on Tuesday said "two soldiers of the caliphate" had targeted the Dallas suburb of Garland.

"We tell ... America that what is coming will be more grievous and more bitter and you will see from the soldiers of the caliphate what will harm you, God willing," said the station, which broadcasts from Raqqa, Syria.

It is the first time IS has claimed responsibility for an attack in the US, though the White House said it was too early to say whether the group had any role.

US government officials have said investigators were looking through the suspects' electronic communications for any evidence of contacts with overseas militants.

Roommates Elton Simpson and Nadir Soofi were shot dead after opening fire with assault rifles outside the Curtis Culwell Center, said authorities.

They hit a security guard in the leg before a police officer returned fire with a pistol, striking and killing both men, who were wearing body armour.

Investigators have been collecting evidence at an apartment complex in Phoenix, Arizona, where both suspects lived in the same unit.

Court documents show that Simpson had been under federal surveillance since 2006 and was convicted in 2011 of lying to FBI agents about his desire to join violent jihad in Somalia.

His family said in a statement released late on Monday they were "heartbroken and in a state of deep shock" at this "act of senseless violence".

Soofi had struggled to adjust to life in the US after moving there as a schoolboy from Pakistan, according to reports.

His mother, Sharon Soofi, told the Dallas Morning News she had no idea he would turn to violence.

She said her son was "raised in a normal American fashion" and "was very politically involved with the Middle East. Just aware of what's going on".

The cartoon contest was organised by the New York-based American Freedom Defence Initiative, which was offering a $10,000 award for the cartoon judged the best.

The organisation is known for campaigning against the building of an Islamic centre a few blocks from the World Trade Center in Manhattan and for buying advertising space in cities across the US criticising Islam.

People attending the event learned of the shooting only after being informed by police.

Bomb squad units were called in to search the suspects' vehicle and the surrounding area.

They detonated several suspicious items found in the car, but no bombs were discovered.

US Homeland Security Secretary Jeh Johnson praised the police officer who gunned down the suspects for having "acted quickly and decisively, and thereby likely (saving) a number of innocent lives".