Texas gunman encouraged by IS supporter: Experts

One of the gunmen who attacked a Prophet Muhammad cartoon competition in Texas was in repeated contact with an American supporter of the Islamic State in Iraq and Syria (ISIS) group, terror experts said.

Mohamed Abdullahi Hassan, an American who joined the radical group ISIS and may be fighting in Somalia, reportedly spoke on Twitter with Elton Simpson before Simpson allegedly joined the attack in the Dallas suburb of Garland and was killed by the police.

Suspected militant gunmen Simpson, 31, and Nadir Soofi, 34, attacked the cartoon contest on Sunday, wounding a security guard before being shot dead in their turn. The US government is probing their radical links.

Hassan is believed to have used the name Mujahid Miski in his discussion of militant attacks with Simpson. These included public tweets and private messages, according to the SITE Intelligence Group, a private watchdog. "Miski was online talking about Charlie Hebdo and how people in the US had to do similar things," terrorism expert David Ibsen, of separate group the Counter Extremism Project, told AFP on Wednesday.

Depictions of Prophet Muhammad are offensive to many Muslims and have prompted attacks in the past.

In January, 12 people were gunned down in an Islamist attack on the French satirical paper Charlie Hebdo in Paris, which had also published cartoons of the Prophet.

The now suspended account believed to belong to Simpson posted and endorsed numerous pro-ISIS messages and communicated with alleged ISIS fighters in the field.

"Upon Simpson's final tweet and news of the attack being announced across media outlets, Miski celebrated Simpson as both a martyr and a friend," SITE director Rita Katz wrote, in an analysis of the militants' communication.

Private messages between the pair showed that Simpson was prepared to go to Syria to fight before carrying out the Texas attack, according to files obtained by the FBI and reported by the ABC television station.

Simpson was sentenced to three years' probation in 2011 after lying to the FBI about plans to travel to Somalia. He was allegedly planning to travel to wage holy war.