Their backgrounds have chilling similarities.

Like the two brothers responsible for the Boston Marathon bombings, the suicide bomber in the Manchester Arena terror attack was a son of refugees who made a brief visit to his parents’ homeland and returned with extreme Islamic views.

He was once known as a pot-smoking party-goer, like the two Tsarnaev brothers responsible for Boston’s mayhem.

As with the Tsarnaev brothers, authorities are questioning whether intelligence officials missed signs of the Manchester bomber’s radicalization.

The striking parallel between the two deadly episodes is “the going abroad and getting access’’ to a terrorist philosophy, said Karen Greenberg, director of the Center on National Security at Fordham University School of Law. “You come back trained, and with an ideological commitment that we want to prevent.

“This is why law enforcement has focused so much on foreign fighters,’’ Greenberg added.

In one key area, there is a difference: unlike the Tsarnaev brothers, who acted alone when they planted two bombs near the finish line of the Boston Marathon in 2013, Salman Abedi appeared to be part of a broader terror cell in England.

The United States has not suffered an attack by a cell since the Sept. 11, 2001, terror attacks, but terrorism analysts say that such networks are a growing concern in Europe and the United States should also be on guard.

In other ways, though, the Tsarnaev brothers and Abedi appear to have followed a similar path. The elder brother, Tamerlan Tsarnaev, who was 26 at the time of the 2013 Marathon bombing, was by many accounts a disgruntled young man who traveled to his family’s native Dagestan, a restive region in Russia, for several months in 2012 and returned with a radical view of Islam.

Back in Cambridge, he shed his flashy clothing for a more modest style of dress and adopted a more religious lifestyle.

According to FBI records introduced in federal court, a relative told federal agents that Tamerlan had expressed interest in joining a terrorist group in Russia, and that it became clear he went to “Russia to fight jihad in the streets.’’ Russian authorities had warned the FBI of Tsarnaev’s radicalization.

Lawyers for Dzhokhar Tsarnaev argued in his trial that Tamerlan corrupted the younger brother, who was a pot-dealing, party-going college student who began to scour terrorist propaganda on the Internet. He hung a black flag with Islamic writing that is often used as a banner for terror groups in his bedroom.

Dzhokhar Tsarnaev credited their support of the terrorist group Al Qaeda as the motive for the bombing.

Abedi, 22, has been described in European press reports as a “fun guy’’ who drank and took drugs and had possible ties to local gangs. Recently, he had traveled to his family’s native Libya, where his parents had fled Moammar Gadhafi’s regime, and he appeared increasingly religious and radical. He also flew a black Islamist flag from his Manchester home.

His actions raised suspicion among British security forces, but they had no cause to arrest him, according to media reports. He dropped out of the University of Salford, where he had been studying business and management, according to European press reports.

Alexander Meleagrou-Hitchens, research director of the Program on Extremism at George Washington University, said the Tsarnaev brothers and Abedi share commonalities with other young terrorists, including the man behind the Nice attack in July 2016, in which a cargo truck was driven directly into crowds, and in Brussels four months earlier, when three coordinated suicide bombers killed 32 people.

“Most cases in the West involve some sort of process involving a changing of ‘who you were before’ to some sort of extreme,’’ he said. “It’s this Western Muslim who wants to rebel against society.’’

However, he said, Abedi’s attack appeared far more sophisticated than the others — the bomb “was made by someone who really knew what they were doing,’’ he said — suggesting Abedi was part of a larger conspiracy. The Islamic State terrorist group has claimed responsibility for the attack.

And while the Tsarnaevs seemed to be isolated from possible terrorist influences in Cambridge, Abedi lived in a neighborhood of South Manchester where at least 16 other people have traveled or attempted to travel to a country known for terrorist activity.

Also, the Times of London reported that Abedi was known to security services as an associate of Islamic State recruiter Raphael Hostey, also from Manchester, who was killed in a drone strike last year in Syria.

“The belief is there is this cluster he was involved with,’’ Meleagrou-Hitchens said.

He and other security analysts said the growth of local cell operations is an increasing concern for law enforcement officials, particularly in Europe, because of open travel and proximity to countries with known terror activities.

The online British newspaper The Independent reported that of at least 850 extremists known to have left the United Kingdom to travel to Iraq and Syria, at least half are believed to have returned to Britain.

“We have to be very cognizant of people who are taking short trips to areas that are identified as troubled areas, places were terrorism and radicalization are being practiced,’’ Edward F. Davis, who was Boston’s police commissioner during the Boston Marathon bombing, said in an interview.

Davis, who now works as a security consultant, said the attacks — including the backgrounds of the bombers and their motives — demonstrate the need for better screening of people who travel to known terrorism hot spots and seek to reenter the country.

“It’s a warning when someone travels to Syria, [to] Dagestan,’’ said Davis, who believes Tamerlan Tsarnaev was taught how to build the bombs used in the Marathon attack when he traveled to Dagestan.

Davis said he was not calling for a travel ban like the one ordered by President Trump that would have halted immigration from six Muslim-majority nations. Civil rights advocates argued that the ban discriminated against Muslims. A federal appeals court in Virginia on Thursday upheld a halt of the order, though the Trump administration said it would appeal to the US Supreme Court.

But, Davis said, law enforcement needs more leeway in screening people coming from countries where there is known terrorism activity if there is any hint of radicalization, saying authorities can no longer wait until there is probable cause to believe someone may carry out an attack.

Matthew Segal, legal director for the American Civil Liberties Union of Massachusetts, cautioned against generalizing about people who come from a specific place.

“Vetting people based on their individual and actual behavior makes sense,’’ he said. “But banning people based on broad assumptions about their religion and nationality doesn’t make sense and is deeply offensive to American values and the United States Constitution.’’

Milton J. Valencia can be reached at milton.valencia-@globe.com. Follow him on Twitter @miltonvalencia.