Sixty-four Americans have travelled to Iraq and Syria to join jihadist groups, according to a new report.

The George Washington University Programme on Extremism’s report, titled “The Travellers: American Jihadists in Syria and Iraq,” included several people whose identities have never been released, as well as 12 people who returned to the US.

The study found that most of the American jihadists were associated with terror group Isis and came mostly from Minnesota, Virginia, and Ohio, but were spread across more than 16 states.

Dr Alexander Melagrou-Hitchens, the Director of the Programme on Extremism and one of the report's authors, told The Independent his team are still looking into why jihadists hail mostly from these places but there is "no obvious reason."

Mr Hitchens said for Minnesota at least, recruiting networks are a "long-standing issue" because of the established Somali-American community, centred around Minneapolis.

The FBI and other law enforcement authorities found that several people had travelled to join up with another terror group called al Shabab in Somalia before Isis really established itself, he explained.

So once Isis became a real threat to the US, it had a "built-in network of recruiters" and "radicalised" people with information, propaganda, and links in the area.

Mr Hitchens also said Ohio has, since the early days after the September 11th attacks, also been a "hotspot" or "recruitment hub" for extremists.

In these and other cases, it may have just been an instance of "serendipitous" connections to radicalised, "charismatic" people within a person's community.

Researchers found a few characteristics in common for these 64 people, however – being male with an average of 27.

But, the authors of the report warned that no single profile, whether demographically or in terms of motivation, could be used to peg these jihadist-travellers.

Mr Hitchens also said that these 64 people are just a sample of those that have travelled and/or returned since information on others is still confidential due to ongoing investigations or for other security reasons. 

As the report stated: “Far fewer foreign volunteers joined up with Isis from the US than from Europe.”

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“The estimated number of foreign volunteers from Europe ranges from 5,000 to 6,000, most of them from France, Belgium, Germany and the United Kingdom, while US officials speak of several hundred,” it said.

That figure – estimated to be 250-300 Americans – included people arrested prior to leaving the country, those arrested abroad, and those known to have been killed.

However, the real numbers remain somewhat unclear. “Most of the 250 to 300 American jihadist recruits mentioned by authorities have not been publicly identified, suggesting ongoing investigations, sealed indictments, and some uncertainty,” Mr Hitchens wrote.

Of the 64 identifiable travellers, 12 had returned to the US by the time the report was completed.

Only one of them was identified by police to have come back with an intent to attack the US.

The sample in the report also included those who were arrested even before joining up with Isis.

Mr Hitchens explained those people have been arrested often at the airport gate just ahead of boarding a flight because they were already under a "reasonable suspicion" by the FBI or other agency.

These people's online activity and personal connections have been monitored for some time prior to their arrest on a charge of "attempt to provide material support" to Isis.

It is rare, but sometimes they could also been reported by their family or community for exhibiting extremist tendencies, at which point an investigation is opened on them.

The centre's research also found something curious in terms of what happens to returnees and those arrested prior to travelling to Iraq or Syria in terms of sentencing.

The average sentence for travellers who returned to the US from being with Isis in those countries was around 10 years. For those who were caught before meeting up with the terror group, it clocked in at 14 years.

The difference can seem counterintuitive, but Mr Hitchens explained it could have come down to intelligence gathering.

The returnees who were moving with Isis have insight into the group's operations, plans, supply levels, locations, and any other details helpful to the US to track and fight Isis as well as prevent future attacks.

This intelligence may have been traded for a lighter sentence.

Another issue is that it is "much harder to prove" what returnees actually did overseas than what those arrested on the attempted charges while still on US soil have done.

While social media groups and Isis’ extensive online presence were part of the equation in how these Americans ended up in the terror group’s ranks, the researchers found that personal networks within the US prior to their trips is really what made the difference.

“More than 85 percent of the travellers in this study had personal connections to other travellers or jihadist supporters in the US prior to leaving for Syria or Iraq,” researchers wrote.