NEW YORK (Reuters) - The online video’s message was clear: Supporters of Islamic State who could not travel overseas to join the militant group should carry out attacks wherever they were in the United States or Europe.

Bangladeshi immigrant Akayed Ullah, 27, followed those instructions on Monday when he tried to set off a homemade bomb in one of New York’s busiest commuter hubs, in an attack that illustrates the difficulty of stopping “do-it-yourself” attacks by radicals who act alone.

While harder to stop than attacks coordinated by multiple people - whose communications may be more easily monitored by law enforcement or intelligence agencies - they also tend to do less damage. Ullah was the person most seriously wounded when his bomb ignited but did not detonate in an underground passageway linking the Port Authority Bus Terminal and the Times Square subway statin; three others sustained lesser injuries.

“They tend to be less organized and less deadly,” said Seamus Hughes, a former adviser at the U.S. government’s National Counterterrorism Center. “That’s because you’re dealing with more, for lack of a better word, amateurs.”

The do-it-yourself style of attack is on the rise in the United States, according to research by the Program on Extremism at George Washington University, where Hughes is deputy director.

The United States has seen 19 attacks perpetrated by Islamic State-inspired people since the group declared a “caliphate” in June 2014 after capturing broad swaths of Iraq and Syria. Of those, 12 occurred in 2016 and 2017, almost twice as many as in the two preceding years.

“You’re going to see continued numbers of plots and, unfortunately, attacks,” Hughes said.

FILE PHOTO: A New York City Police (NYPD) officer stands in the subway corridor, at the New York Port Authority subway station near the site of an attempted detonation the day before, in New York City, U.S. December 12, 2017. REUTERS/Brendan McDermid/File Photo

Ullah began immersing himself in Islamic State propaganda as early as 2014, three years after he arrived in the United States as a legal immigrant, according to federal prosecutors who charged him with terrorism offences. They said in court papers that Ullah’s computer records showed that he viewed ISIS videos urging supporters of the group to launch attacks where they lived.

Experts said the success of Western allies in retaking most of Islamic State’s territory could inspire more attacks out of anger or vengeance.

“No group has been as successful at drawing people into its perverse ideology as ISIS,” Federal Bureau of Investigation Director Christopher Wray said in congressional testimony last week. “Through the internet, terrorists overseas now have access into our local communities to target and recruit our citizens.”

National security analysts generally divide such perpetrators into three broad categories.

Some attackers act at the direction of a group, like the Islamic State-backed militants who carried out coordinated attacks in Paris in 2015, killing 130; others have some limited contact with an organization but act largely on their own. A third type has no communication with a group but engage in violence after being radicalized by online propaganda.

It is easier for trained, battle-hardened ISIS fighters to travel from the Middle East to Europe than for them to reach the United States. That helps explain why U.S. attacks have largely been the work of “self-made” terrorists, said Brandeis University professor and radicalization expert Jytte Klausen.

“In these recent cases, we’ve seen very few indications that there was any type of direct training,” Klausen said.

Self-directed perpetrators are the hardest for investigators to identify. Their ranks appear to include Ullah, as well as two other recent New York attackers: Ahmad Rahimi, the man who injured 30 with a homemade bomb in Manhattan in September 2016, and Sayfullo Saipov, the Uzbek immigrant accused of killing eight by speeding a rental truck down a bike lane in October.

While that type of attacker typically is less destructive, there are important exceptions. Oklahoma City bomber Timothy McVeigh and Terry Nichols killed 168 people, and Omar Mateen gunned down 49 people at a gay nightclub in Orlando last year.

“A single individual or two can still create a lot of damage,” said Max Abrahms, a professor at Northeastern University who studies terrorism. “But they’re not able to wage sustained terrorist campaigns.”