Islamic State (IS) has claimed responsibility for an attack on a Prophet Muhammad cartoon contest in Texas.
The crime scene before the removal of the suspects' bodies outside of the Curtis Culwell Center in Garland, Texas. Photo: AFP
It said that "two soldiers of the caliphate" carried out the attack at a conference centre near Dallas.
IS's al-Bayan Radio news bulletin said the exhibition "was portraying negative pictures of the Prophet Muhammad".
Both suspects were shot dead after opening fire at the contest on Sunday.
Correspondents say it is believed to be the first time that IS has claimed to have carried out an attack in the US.
"We tell America that what is coming will be even bigger and more bitter, and that you will see the soldiers of the Islamic State do terrible things," the statement released by the group said.
A White House spokesperson said it was too early to link the group to the attack, which is still under investigation.
Terror suspect
Court documents have shown that one of two gunmen shot dead had been a terror suspect.
Elton Simpson had been under surveillance since 2006 and was convicted in 2010 for lying to FBI agents about plans to go to Somalia to engage in violent jihad, or holy war, the files reveal.
A judge found him guilty of making a false statement and he was sentenced to three years' probation and a $US600 fine.
He had shared a flat in Arizona with the person named by officials as the other alleged gunman, Nadir Soofi.
On Monday, FBI agents searched their home in Phoenix and a white van parked outside.
The court documents also showed that Simpson had been under investigation since 2006 because of his association with an individual the FBI believed was trying to set up a terrorist cell in Arizona.
He had worshipped at the Islamic Community Center of Phoenix for years but the imam there told the BBC he had not seen him there as much since 2010.
Simpson's family have expressed disbelief that he could have been involved. His father Dunston told ABC News: "What my son did reflects very badly on my family."
The incident on Sunday unfolded when a car drove into the car park of the Curtis Culwell Center in Garland, a city near Dallas, where a Muhammad Art Exhibition organised by the controversial American Freedom Defense Initiative (AFDI) was being held.
The conference included a contest that offered a $US10,000 prize for a cartoon of the Prophet Muhammad. Depictions of Muhammad are offensive to many Muslims.
The two men in the car opened fire with assault rifles on two security officers, but were shot dead on the spot.
Social media campaign
Meanwhile, an Australian Twitter user who recommended attacking the exhibition just days before the shoorings has now posted a guide on how to join Islamic State.
The user, known on Twitter as 'Australia Witness' shared a map of the community centre where the event was being held.
Neil Fergus is the chief executive of security consultancy Intelligent Risks - he worked for federal security agencies for 20 years and was the intelligence director for the 2000 Olympics in Sydney.
He told Morning Report this was yet another example of how IS has been using social media to reach across traditional state boundaries.
Listen to Neil Fergus
-BBC/RNZ