Editor's note: Peter Bergen is CNN's national security analyst, a vice president at the New America Foundation and the author of "Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad." David Sterman is a research associate at the New America Foundation. This article is adapted from a commentary that appeared earlier this month.
(CNN) -- Douglas McAuthur McCain grew up in the Minneapolis area. Aged 33, he died more than 6,000 miles to the east of his birthplace, fighting in Syria for ISIS, the group that calls itself the "Islamic State."
ISIS is a group that even al Qaeda has rejected, in part, for its reprehensible tactics, which run to public crucifixions.
McCain's transformation to a militant jihadist left his family "devastated," his uncle, Ken McCain, told CNN.
McCain is the first American known to have been killed while fighting for ISIS. But he is not the only American drawn to the group or to al Qaeda's affiliate in Syria, the al-Nusra Front.
In May 2014, Moner Abu-Salha, a 22-year-old American citizen from Florida, conducted a suicide bombing on behalf of Nusra. Abu-Salha had reportedly re-entered the United States after he received training from the group in Syria and before returning to the conflict.
Abu-Salha is the first American suicide bomber known to have died in Syria.
Some 100 other Americans are believed to have either fought in Syria since 2011 or been arrested before they could get there.
According to a count by the New America Foundation, eight of these individuals have been indicted for traveling, attempting to travel, or facilitating the travel of others to fight with ISIS or the al-Nusra Front.
Some of these cases involved those who tried to join ISIS or Nusra, but were arrested before they could leave the States:
• Abdella Tounisi, an 18-year-old American citizen from Aurora, Illinois, was arrested and charged with attempting to provide material support to Nusra. On April 19, 2013, he was caught in a sting operation and said that he had no combat skills to speak of: "Concerning my fighting skills, to be honest, I do not have any." Tounisi pleaded not guilty and awaits trial.
• Basit Sheikh, a North Carolina man, was arrested in November 2013 at Raleigh-Durham International Airport in North Carolina while allegedly trying to fly to Lebanon in order to join Nusra. He awaits trial.
• Nicholas Teausant, a 20-year-old from California, was arrested in March 2014 while allegedly traveling to join ISIS. Teausant pleaded not guilty and awaits trial.
• Shannon Conley, a 19-year-old Denver woman, was arrested in April 2014 and charged with attempting to provide material support to ISIS. She awaits trial.
• Adam Dandach, a 20-year-old Orange County man, was arrested at John Wayne Airport while allegedly attempting to travel to join ISIS. Dandach was charged with lying about needing a passport replacement to conceal that the real reason he needed to replace his passport was that his mother had hidden his original passport to prevent his travel. He awaits trial.
• In December, Sinh Vinh Ngo Nguyen, an American citizen from southern California, pleaded guilty to a charge of attempting to provide material support to al Qaeda. Between December 2012 and April 2013, Nguyen had traveled to Syria, where, he stated, he fought alongside Nusra. On his return, Nguyen discussed with an informant his intent to participate further in jihad.
• In August 2013, Gufran Mohammed, a naturalized American citizen living in Saudi Arabia, was charged with attempting to provide material support to Nusra in Syria by facilitating the recruitment of experienced fighters from al Qaeda's Somali affiliate to travel to Syria.
• Michael Todd Wolfe, a 23-year-old Texas man, pleaded guilty to attempting to provide material support to a foreign terrorist organization by traveling to Syria to fight with ISIS.
Because of the murky nature of the Syrian conflict, it is sometimes a challenge to identify the groups that Americans traveling to Syria are involved with.
Two cases -- those of Nicole Mansfield and Eric Harroun -- illustrate this difficulty. Mansfield, a Michigan woman, was killed in Syria in 2013, reportedly during a military clash. However, details on whether she was fighting in Syria and, if so, which group she fought for remain under investigation.
Harroun was indicted upon his return to the States in 2013 for fighting with Nusra, but it was later discovered that the FBI had mistranslated the name of the group he fought with and he had actually fought with a group that was not aligned with al Qaeda.
So far, no U.S. citizen involved in fighting or supporting Nusra or ISIS has been charged with plotting to conduct an attack inside the United States.
Further, ISIS' predecessor, al Qaeda in Iraq, never tried to conduct an attack on the American homeland, although it did bomb three American hotels in Jordan in 2005.
And it's also worth noting that in none of the successful terrorist attacks in the States since 9/11 -- such as the Boston Marathon bombings last year or Maj. Nidal Hasan's massacre at Fort Hood, Texas, in 2009 -- did any of the convicted or alleged perpetrators receive training overseas.
Returning foreign fighters from the Syrian conflict pose a far greater threat to Europe, which has contributed a much larger number of foreign fighters to the conflict than the United States, including an estimated 700 from France, 450 from the United Kingdom and 270 from Germany.
Unlike in the United States, European countries have reported specific terrorist plots tied to returning Syrian fighters. Mehdi Nemmouche, a suspect in the May 24 shootings at a Jewish museum in Brussels, Belgium, that killed four people, spent about a year with jihadist fighters in Syria, according to the Paris prosecutor in the case. But Nemmouche's case is the only instance of lethal violence by a returning Syrian fighter in the West.
What can be done? Western governments are keenly aware of the problem of Syrian veterans coming home both radicalized and trained. The problem is that in some European countries with hundreds of returnees, it is just not possible to monitor all of them. That was vividly illustrated by the case of Nemmouche.
Information-sharing between Western governments about the identities of those who have traveled to Syria and have received militant training is the key to preventing more incidents such as the one at the Jewish Museum in Brussels.