As investigators probe the background of the two gunmen shot dead in an attack on a Texas competition of cartoons of the Prophet Mohammed, online links are emerging to a British Islamic (Isis) State jihadist believed to be the group's hacking and propaganda kingpin.

Jihadist group Isis, which controls parts of Syria and Iraq, claimed responsibility for the attack, but offered no evidence for its involvement.

"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 future is just around the corner," the group announced on its radio station.

But a web of connections is emerging on social media between gunman Elton Simpson and supporters of the jihadist group, and at its centre is British jihadist Junaid Hussain.

On Twitter, Simpson allegedly urged people to follow the account of Abu Hussain al-Britani, which experts believe is Hussain's online moniker.

Hussain in turn hailed the Texas attacks.

"Allahu Akbar!!!! 2 of our brothers just opened fire" he tweeted. "They Thought They Was Safe In Texas From The Soldiers of The Islamic State".

"If there is no check on the freedom of your speech, then let your hearts be open to the freedom of our actions," he continues.

The old account appears to have been closed down in the wake of the attacks, but under a new account he tweeted "You ain't seen nothing yet".

Originally from Birmingham, Hussain, who is in his early 20s, escaped to Syria in 2013 while on police bail, after allegedly hacking personal information of former Prime Minister Tony Blair.

Authorities believe that Hussain is the mastermind behind the jihadist group's cyber campaigns, including the hacking of the US Central Command's Twitter account last year.

From the account featuring a picture of a masked man holding a Kalashnikov rifle, experts believe that Hussain has tweeted a series of messages imploring others to join the jihadist group and urging violence against the West.

Simpson, it emerged, had been known to US authorities since 2006, when he was in contact with a man attempting to form a terror cell in Arizona, and in 2011 was found guilty of lying over plans to travel to Somalia to wage jihad.

Authorities are now investigating whether the two were in direct contact, with counter-terrorism officials having long argued that instead of forming terror cells on the model of al-Qaeda to commit attacks in the West, Isis seeks to inspire sympathisers to carry out their own 'lone wolf' attacks with its online propaganda.

"They're developing that with a very, very determined approach to exploiting social media and propaganda and the like," Scotland Yard Assistant Commissioner Mark Rowley recently said.

"The real aim being to kill and get footage of that to propagate the message and propagate themselves to their cause," he added.