WASHINGTON — Four months after an Islamic State suicide bomber killed scores of people, including 13 American service members, outside the airport in Kabul, Afghanistan, U.S. and foreign intelligence officials have pieced together a profile of the assailant.

Military commanders say they are using that information to focus on an Islamic State cell that they believe was involved in the attack, including its leadership and foot soldiers. The cell members could be among the first insurgents struck by armed MQ-9 Reaper drones flying missions over Afghanistan from a base in the Persian Gulf. The United States has not carried out any airstrikes in the country since the last American troops left on Aug. 30.

The attack at the airport’s Abbey Gate unfolded four days earlier, during the frenzied final days of the largest noncombatant evacuation ever conducted by the U.S. military. It was one of the deadliest attacks of the 20-year war in Afghanistan.

The Islamic State identified the suicide bomber as Abdul Rahman Al-Logari. American officials say he was a former engineering student who was one of several thousand militants freed from at least two high-security prisons after the Taliban seized control of Kabul on Aug. 15. The Taliban emptied the facilities indiscriminately, releasing not only their own imprisoned members but also fighters from Islamic State Khorasan, or ISIS-K, the group’s branch in Afghanistan and the Taliban’s nemesis.

“It’s hard to explain what the thinking was in letting out people who were a threat to the Taliban,” Edmund Fitton-Brown, a senior U.N. counterterrorism official, said at a recent security conference in Doha, Qatar.

Mr. Logari was not unknown to the Americans. In 2017, the C.I.A. tipped off Indian intelligence agents that he was plotting a suicide bombing in New Delhi, U.S. officials said. Indian authorities foiled the attack and turned Mr. Logari over to the C.I.A., which sent him to Afghanistan to serve time at the Parwan prison at Bagram Air Base. He remained there until he was freed amid the chaos after Kabul fell.

Eleven days later, on Aug. 26 at 5:48 p.m., the bomber, wearing a 25-pound explosive vest under his clothing, walked up to a group of American troops who were frisking those hoping to enter Hamid Karzai International Airport. He waited, military officials said, until just before he was about to be searched before detonating the bomb, which was unusually large for a suicide vest, killing himself and nearly 200 others.

The attack raised ISIS-K’s international profile, and positioned it both as a major threat to the Taliban’s ability to govern the country and, according to American officials, as the most imminent terrorist risk to the United States coming out of Afghanistan.

“The group has gained some notoriety in a way that could be quite compelling for them on the transnational stage,” Christine Abizaid, the director of the National Counterterrorism Center, said in October at a national security conference in Sea Island, Ga. “At the same time, they’re fighting the Taliban. How that force-on-force engagement in Afghanistan will go will have some defining characteristics about what the transnational threat looks like.”

In October, Colin H. Kahl, the under secretary of defense for policy, told the Senate Armed Services Committee that ISIS-K could be able to attack the United States sometime in 2022. “We could see ISIS-K generate that capability in somewhere between six and twelve months,” he said.

The Parwan prison at Bagram and the Pul-e-Charkhi prison near Kabul were the Afghan government’s two main high-security prisons. The United States built Parwan in 2009 and transferred it to Afghan government control three years later.

On July 1, with little warning and no public ceremony, U.S. forces abandoned Bagram Air Base, the main hub for American military operations. Six weeks later, on Aug. 15, Taliban fighters swept into the base and threw open the prison gates.

The Taliban killed one prominent prisoner — a former top leader of the Islamic State in Afghanistan, Omar Khalid Khorasani — and released more than 12,000 others, including roughly 6,000 Taliban, 1,800 ISIS-K and nearly three dozen Qaeda fighters, according to U.S. officials.

“The fiasco in Afghanistan has put hundreds of terrorists back on the street,” said Bruce O. Riedel, a former C.I.A. officer who ran President Barack Obama’s first Afghanistan policy review.

One of them was Mr. Logari, the son of an Afghan merchant who frequently visited India and Pakistan for business. Mr. Logari moved to India in 2017 to study engineering at Manav Rachna University near New Delhi.

Recruited by ISIS-K, Mr. Logari was arrested in relation to the New Delhi plot and handed over to the C.I.A. by India’s foreign spy service, the Research and Analysis Wing, in September 2017, according to Indian media reports that were confirmed by American officials. A C.I.A. spokesman declined to comment.

Mr. Logari spent time in both the Pul-e-Charki and Parwan prisons, American officials said, but it is unclear how he linked up with the ISIS-K attack cell in Kabul, or why and how he came to be the Abbey Gate bomber.

Soon after the attack, however, American officials and U.S. media reports disclosed that the bomber had been released from prison just days earlier.

The Islamic State seized on the spectacular nature of the bombing, boasting about its size, location and timing in social media posts, according to the SITE Intelligence Group, which monitors jihadist media.

In the months since the attack, U.S. intelligence analysts and military officials say they have focused on learning more about the ISIS-K strike cell, and any future attacks it may be plotting against the West.

Created six years ago by disaffected Pakistani Taliban fighters, the group’s ranks fell to about 1,500 to 2,000 fighters last year, about half that of its peak in 2016 before U.S. airstrikes and Afghan commando raids took a toll, killing many of its leaders. In June 2020, an ambitious new commander, Shahab al-Muhajir, took over the group and has been trying to recruit Taliban fighters and other militants.

Even before the Abbey Gate bombing, ISIS-K had vastly increased the pace of its attacks in 2021, a United Nations report concluded in June.

The violence has strained Afghanistan’s new and untested government and raised red flags in the West about the group’s potential resurgence.

President Biden and his top commanders have said the United States would carry out “over-the-horizon” strikes from a base in the United Arab Emirates against ISIS and Qaeda insurgents who threaten the United States.

Gen. Kenneth F. McKenzie Jr., the head of the military’s Central Command, said in early December that the departure of the U.S. military and intelligence assets from Afghanistan had made tracking the groups harder but “not impossible.”

Mujib Mashal contributed reporting from New Delhi.