Texas attack
Investigators at the scene of the attack in Garland, Texas, in which two gunmen were shot deadGetty Images

Islamic State (Isis) has claimed responsibility for an attack on a Mohammed cartoons contest in Texas in which two gunmen were shot dead.

The jihadist group, which controls large areas of Syria and Iraq, made the announcement via its official radio station, reports AFP.

"Two of the soldiers of the caliphate executed an attack on an art exhibit in Garland, Texas, and this exhibit was portraying negative pictures of the Prophet Mohammed," the terrorist group said.

"We tell America that what is coming will be even bigger and more bitter, and that you will see the soldiers of Isis do terrible things," the group announced.

It is the first time the group has claimed responsibility for an attack on US soil.

It emerged yesterday that suspected gunman Elton Simpson had tweeted in support of IS before Sunday's attack (3 May), and that investigators were probing whether the pair had links to foreign terror groups.

"May Allah accept us as the mujahideen," said a tweet that officials believe was posted by Simpson, adding that he and his fellow alleged attacker, Nadir Soofi, had pledged loyalty to "Amirul Mu'mineen" (the leader of the faithful) – who terrorism experts told CNN could refer to IS leader Abu Bakr al-Baghdadi.

An account believed by terrorism monitoring groups to belong to British IS jihadist Junaid Hussain praised the attacks.

"If there is no check on the freedom of your speech, then let your hearts be open to the freedom of our actions," wrote Hussain, whose tweets under the name Abu Hussain al-Britani.

Previously, Simpson had urged people to follow Hussain's account.

Simpson had been on the radar of security services since 2006, and was convicted in 2011 of lying to an FBI agent about plans to travel to Somalia to wage jihad.

In the attack, the men drove up to the conference centre in Garland, Texas, where the controversial event was hosted, and opened fire. Both were shot dead by a police officer.

In its propaganda, IS had repeatedly pledged to target the West.

The American Freedom Defense Initiative said it organised the cartoons competition in defence of free speech.