Five years ago this month, an American-backed Kurdish and Arab militia ousted Islamic State fighters from a village in eastern Syria, the group’s last sliver of territory.

Since then, the organization that once staked out a self-proclaimed caliphate across Iraq and Syria has metastasized into a more traditional terrorist group — a clandestine network of cells from West Africa to Southeast Asia engaged in guerrilla attacks, bombings and targeted assassinations.

None of the group’s affiliates have been as relentless as the Islamic State in Khorasan, which is active in Afghanistan, Pakistan and Iran and has set its sights on attacking Europe and beyond. U.S. officials say the group carried out the attack near Moscow on Friday, killing scores of people and wounding many others.

In January, Islamic State Khorasan, or ISIS-K, carried out twin bombings in Iran that killed scores and wounded hundreds of others at a memorial service for Iran’s former top general, Qassim Suleimani, who was targeted in a U.S. drone strike four years earlier.

“The threat from ISIS,” Avril D. Haines, the director of national intelligence, told a Senate panel this month, “remains a significant counterterrorism concern.” Most attacks “globally taken on by ISIS have actually occurred by parts of ISIS that are outside of Afghanistan,” she said.

Gen. Michael E. Kurilla, the head of the military’s Central Command, told a House committee on Thursday that ISIS-K “retains the capability and the will to attack U.S. and Western interests abroad in as little as six months with little to no warning.”

American counterterrorism specialists on Sunday dismissed the Kremlin’s suggestion that Ukraine was behind Friday’s attack near Moscow. “The modus operandi was classic ISIS,” said Bruce Hoffman, a terrorism scholar at the Council on Foreign Relations.

The assault was the third concert venue in the Northern Hemisphere that ISIS has struck in the past decade, Mr. Hoffman said, following an attack on the Bataclan theater in Paris in November 2015 (as part of a broader operation that struck other targets in the city) and a suicide bombing at an Ariana Grande concert in Manchester Arena, England, in May 2017.

Islamic State Khorasan, founded in 2015 by disaffected members of the Pakistani Taliban, burst onto the international jihadist scene after the Taliban toppled the Afghan government in 2021. During the U.S. military withdrawal from the country, ISIS-K carried out a suicide bombing at the international airport in Kabul in August 2021 that killed 13 U.S. service members and as many as 170 civilians.

Since then, the Taliban have been fighting ISIS-K in Afghanistan. So far, the Taliban’s security services have prevented the group from seizing territory or recruiting large numbers of former Taliban fighters, according to U.S. counterterrorism officials.

But the upward arc and scope of ISIS-K’s attacks have increased in recent years, with cross-border strikes into Pakistan and a growing number of plots in Europe. Most of those European plots were thwarted, prompting Western intelligence assessments that the group might have reached the lethal limits of its capabilities.

Last July, Germany and the Netherlands coordinated arrests targeting seven Tajik, Turkmen and Kyrgyz individuals linked to a ISIS-K network who were suspected of plotting attacks in Germany.

Three men were arrested in the German state of North Rhine-Westphalia over alleged plans to attack the Cologne Cathedral on New Year’s Eve 2023. The raids were linked to three other arrests in Austria and one in Germany on Dec. 24. The four people were reportedly acting in support of ISIS-K.

American and other Western counterterrorism officials say these plots were organized by low-level operatives who were detected and thwarted relatively quickly.

“Thus far, ISIS-Khorasan has relied primarily on inexperienced operatives in Europe to try to advance attacks in its name,” Christine S. Abizaid, the head of the National Counterterrorism Center, told a House committee in November.

But there are worrisome signs that ISIS-K is learning from its mistakes. In January, masked assailants attacked a Roman Catholic church in Istanbul, killing one person. Shortly afterward, the Islamic State, through its official Amaq News Agency, claimed responsibility. Turkish law enforcement forces detained 47 people, most of them Central Asian nationals.

Since then, Turkish security forces have launched mass counteroperations against ISIS suspects in Turkey, Syria and Iraq. Several European investigations shed light on the global and interconnected nature of ISIS finances, according to a United Nations report in January, which identified Turkey as a logistical hub for ISIS-K operations in Europe.

The Moscow and Iran attacks demonstrated more sophistication, counterterrorism officials said, suggesting a greater level of planning and an ability to tap into local extremist networks.

“ISIS-K has been fixated on Russia for the past two years,” frequently criticizing President Vladimir V. Putin in its propaganda, said Colin P. Clarke, a counterterrorism analyst at the Soufan Group, a security consulting firm based in New York. “ISIS-K accuses the Kremlin of having Muslim blood in its hands, referencing Moscow’s interventions in Afghanistan, Chechnya and Syria.”

A significant portion of ISIS-K’s members are of Central Asian origin, and there is a large contingent of Central Asians living and working in Russia. Some of these individuals may have become radicalized and been in position to serve in a logistical function, stockpiling weapons, Mr. Clarke said.

Daniel Byman, a counterterrorism specialist at Georgetown University, said that “ISIS-K has gathered fighters from Central Asia and the Caucasus under its wing, and they may be responsible for the Moscow attack, either directly or via their own networks.”

Russian and Iranian authorities apparently did not take seriously enough public and more detailed private American warnings of imminent ISIS-K attack plotting, or were distracted by other security challenges.

“In early March, the U.S. government shared information with Russia about a planned terrorist attack in Moscow,” Adrienne Watson, a spokeswoman for the National Security Council, said on Saturday. “We also issued a public advisory to Americans in Russia on March 7. ISIS bears sole responsibility for this attack. There was no Ukrainian involvement whatsoever.”

Russian authorities on Saturday announced the arrest of several suspects in Friday’s attack. But senior American officials said on Sunday that they were still digging into the background of the assailants and trying to determine whether they were deployed from South or Central Asia for this specific attack or if they were already in the country as part of the network of supporters that ISIS-K then engaged and encouraged.

Counterterrorism specialists voiced concern on Sunday that the attacks in Moscow and Iran might embolden ISIS-K to redouble its efforts to strike in Europe, particularly in France, Belgium, Britain and other countries that have been hit on and off for the past decade.

The U.N. report, using a different name for Islamic State Khorasan, said “some individuals of North Caucasus and Central Asian origin traveling from Afghanistan or Ukraine toward Europe represent an opportunity for ISIL-K, which seeks to project violent attacks in the West.” The report concluded that there was evidence of “current and unfinished operational plots on European soil conducted by ISIL-K.”

A senior Western intelligence official identified three main drivers that could inspire ISIS-K operatives to attack: the existence of dormant cells in Europe, images of the war in Gaza and support from Russian-speaking people living in Europe.

One major event this summer has many counterterrorism officials on edge.

“I worry about the Paris Olympics,” said Edmund Fitton-Brown, a former top U.N. counterterrorism official who is now a senior adviser to the Counter Extremism Project. “They would be a premium terrorist target.”