'Terrorism has gone viral': Experts urge greater monitoring of homegrown extremists inspired by IS

Washington's intelligence leaders flooded US television studios over the weekend to warn of the dangers of homegrown terrorism in a concerted push that coincided with a looming deadline to continue the domestic surveillance powers of the National Security Agency.

Seizing on last week's failed attack on a Texas contest to draw cartoons of the Prophet Mohammed, the chairmen of three congressional security committees, the former CIA director and the secretary for homeland security on Sunday all urged greater scrutiny of domestic extremists they claim have been inspired by the Islamic State group.

"Terrorism has gone viral," said Michael McCaul, the Republican chairman of the House homeland security committee, telling there were "thousands of people in the United States who will take up this call to arms when ISIS sends out an internet missive, a tweet, to launch a terror act like we saw in Garland in Texas".

"We are very definitely in a new phase in the global terrorism threat where the so-called lone wolf could strike at any moment," said Jeh Johnson, Barack Obama's secretary for homeland security, on ABC.

Former CIA director Michael Hayden told CNN: "I think the tide's coming in and we're going to see more of what we saw in Texas last week."

Little evidence has yet emerged to support the Islamic State group's claims of direct responsibility for the Garland attack, in which the two gunmen were the only people killed, although FBI director James Comey has argued that the distinction between inspiring and directing attacks is "irrelevant", claiming social media propaganda meant: "It's almost as if there is a devil sitting on the shoulder saying, 'kill, kill, kill,' all day long."