Asked if she knew if American government agencies were monitoring his activity she said they struggled to keep track of American activity, let alone follow global chatter. Investigators box up an assault weapon outside the Curtis Culwell Centre in Garland, Texas, after two men were killed in an attack on a provocative contest for Prophet Mohammed cartoons. Credit:AP "The Australians are having problems following their own guys, you want [the Americans] to follow them too?," she told Fairfax Media. "This is a whole totally new war and it is being fought on Twitter. "There are thousands of new accounts being created every day."

The FBI, CIA and Garland Police have not yet responded to requests for comment. The bodies of gunmen Elton Simpson and Nadir Soofi are removed from behind a car during an investigation by the FBI and local police in Garland, Texas. Credit:Reuters The SITE Intelligence Group is based just outside Washington, DC, tracks terrorist networks and activity online and through social media. Ms Katz has acted as a consultant for US government agencies and foreign governments and testified before Congress on online terrorist activities. "Australi Witness" posts as a "Soldier of #IS and advocate of #Sharia4Australia" and was one of several social media accounts to call for an attack on the Texas community centre hosting an exhibition of cartoons of Prophet Muhammad. An aerial view shows the car that was used the previous night by two gunmen, who were killed by police. Credit:Reuters

The Australian In turn some of his posts were shared by an American man identified on his Twitter feed as "Sharia is Light", who claimed credit for the attack before it happened, describing himself and an accomplice as "mujahideen". An image provided by Site Intelligence Group linking one of the Garland gunman to an Australian Twitter account. Soon after the attack, Australi Witness tweeted:

And fours after that, posted: Australian authorities are looking intently at whether the Twitter user behind "Australi Witness" has broken new laws passed last year by the Abbott government, Fairfax Media understands. Last year, the federal Parliament passed new laws against advocating terrorism, which broadened the circumstances in which prosecutions can be launched against people who encourage, promote and urge extremist violence. Under the changes, prosecutors no longer have to prove that the person who carried out the attack was directly influenced by the advocate, as was previously the case under the narrower definition of "incitement". Rather, the advocacy itself is an offence.

The change aimed to close a gap in the law that became conspicuous through social media platforms such as Twitter. One prominent case involved the well-known extremist preacher Musa Cerantonio, who exhorted his thousands of Twitter followers to violence but was not liable for prosecution because it was too difficult to prove that any one of his followers had carried out an attack because of his tweets. Any prosecution against "Australi Witness" would likely have to tackle complicated questions including whether sharing tweets posted by other users constitutes an endorsement of those messages, and how that sharing relates to tweets posted by the user himself. The event, in which attendees were to present cartoons of Muhammad in competition for a $US10,000 ($12,745) prize, took place in the suburb of Garland, outside Dallas, on Sunday, local time, Two men armed with assault rifles drove up to the building and opened fire, wounding a security guard, Bruce Joiner. An officer with the Garland Police returned fire with a handgun, killing both men. "He did a very good job, and probably saved lives," a Garland Police spokesman said at a news conference on Monday.

Authorities have now identified one of the men as Elton Simpson, 30, of Phoenix, Arizona. The second man has been reported to be his flat-mate, Nadir Soofi. Mr Simpson faced charges in 2010 of travelling to Somalia to engage in "violent jihad" but was only convicted of lying to an agent and was sentenced to three years' probation, The New York Times reported.