THE fatal explosions in Boston have hints of a right-wing terrorist attack rather than al-Qaeda-inspired extremism, according to one of the world's leading experts on counter-terrorism.

Richard Barrett, the former United Nations co-ordinator for the al-Qaeda and Taliban monitoring team, said it was too early to say who was to blame for the marathon blasts.

But Barrett, who has served with MI5, MI6 and the British Foreign Office, said the timing of the attack on Patriots' Day and the relatively small size of the devices suggested the work of a domestic extremist.

His comments came after US supercop Bill Bratton, a former head of Boston police who is now based in London, warned there are "no shortage of potential suspects" behind the explosions.

Barrett, who is now senior director at the Qatar International Academy for Security Studies (QIASS), said: "At the moment it looks more likely that it was a right-wing terrorist incident, rather than an al Qaeda attack because of the size of the devices."

He added: "This happened on Patriots' Day, it is also the day Americans are supposed to have their taxes in, and Boston is quite a symbolic city. These are all little indicators."

Barrett, who is also a former member of a UN task force for promoting global counter-terrorism strategy, said behind the scenes a "very intense" investigation will be unfolding.

"In addition, security arrangements for other events will be quickly reviewed," he said.

"There is Margaret Thatcher's funeral tomorrow and the London Marathon on Sunday. However, there are thousands of these events coming up all the time."

Barrett said the number of right-wing extremist incidents in the US since the September 11 attacks was quite high.

From 2002 to 2007, nine right-wing extremists were indicted for their roles in politically motivated murders.

But between 2008 and 2012, the number mushroomed to 53, according to figures by the New America Foundation.

Before this period, domestic terrorist Eric Rudolph attacked a number of sites including the Centennial Olympic Park in Atlanta in the name of an anti-abortion and anti-gay agenda.

And in 1995, right-winger Timothy McVeigh detonated a truck bomb in front of the Alfred P. Murrah Federal Building in Oklahoma City, killing 168 people and injuring more than 800.

Barrett said al-Qaeda attacks normally involve terrorists who have trained using instructions from the internet or at a training camp, which usually helps identify them.

In contrast, domestic terrorists operate in isolation or through a small number of acquaintances and often have smaller targets in mind.

"That's why many al-Qaeda terrorists have been thwarted - they're too ambitious," he said.

Originally published as Expert blames right-wing terrorists