WASHINGTON — Some time last year, a Kuwaiti man in his early 30s who had spent more than a decade hiding from the American government arrived in northwest Syria, where he met up with other members of Al Qaeda who had begun putting down roots in a country torn by two years of death and chaos.
American intelligence officials believe that the Kuwaiti, known sometimes as Muhsin al-Fadhli, had been sent from Pakistan by Ayman al-Zawahri, Al Qaeda’s leader, to take over a cell that could one day use Syria as a base for attacks in Europe and possibly the United States.
Unlike other jihadist groups that have come to prominence in recent years, the cell that Mr. Fadhli came to lead — known within intelligence and law enforcement agencies as the Khorasan Group — avoided the spotlight. It put out no slick Internet magazines and did not boast of its plans on Twitter.
The group’s evolution from obscurity to infamy has been sudden: The first time President Obama publicly mentioned the group was on Tuesday, when he announced he had ordered an airstrike against it to disrupt what American officials said was a terror plot aimed at the West.
The United States government has yet to confirm whether Mr. Fadhli died in the strike, and American officials have given differing accounts about just how close the group was to mounting an attack, and about what chance any plot had of success. One senior American official on Wednesday described the Khorasan plotting as “aspirational” and said that there did not yet seem to be a concrete plan in the works.
The focus on the Khorasan Group in recent days has, at least for the moment, diverted attention from the Islamic State, the militant group whose recent battlefield successes were Mr. Obama’s original reason for launching airstrikes. It has also underscored the enduring relevance of Al Qaeda’s leadership apparatus in Pakistan, a group that Mr. Obama told the United Nations on Wednesday had been badly battered.
“There’s a contradiction here,” said Bruce O. Riedel, a former C.I.A. analyst now at the Brookings Institution. “If they are that decimated, why are we so alarmed when we detect new evidence of their activities?”
The paucity of public information about the Khorasan Group makes it hard to draw firm conclusions about its ultimate goals. Intelligence officials and terrorism experts believe that the group, although based in Syria, answers ultimately to Mr. Zawahri and Al Qaeda’s core leadership in Pakistan. They said that its size seemed to fluctuate, but that it consisted of approximately two dozen operatives, most of whom came to Syria from Pakistan and Afghanistan beginning in 2012.
Once they arrived in Syria, members of the group established contacts with fighters from the Nusra Front, a Syrian rebel organization that is Al Qaeda’s official affiliate in Syria and one of myriad groups that formed in recent years to fight the government of Syria’s president, Bashar al-Assad. While the Nusra Front remains primarily committed to fighting government troops for territory in Syria’s vicious civil war, analysts said, the Khorasan Group’s focus is on external attacks.
“What core Al Qaeda wanted was some forward-deployed people in Syria, which is an important battlefield for them because it is so close to Europe,” said Seth G. Jones, a terrorism expert at the RAND Corporation.
Several of Mr. Obama’s aides said Tuesday that the airstrikes against the Khorasan operatives were launched to thwart an “imminent” terrorist attack, possibly using concealed explosives to blow up airplanes. But other American officials said that the plot was far from mature, and that there was no indication that Khorasan had settled on a time or location for the attack — or even on the exact method of carrying out the plot.
Some experts said it was more likely that American spy agencies had developed specific intelligence about the location of Mr. Fadhli and others, and that Mr. Obama had ordered the strike to kill the Khorasan operatives before they could scatter.
“Actionable intelligence is really hard to get your hands on, and I suspect that’s what really drove the timing of the strike,” Mr. Riedel said.
Fearing possible reprisal attacks, the Department of Homeland Security issued a warning this week to law enforcement agencies to be on heightened alert for so-called lone-wolf terrorist attacks inside the United States. Speaking to reporters on Wednesday after visiting an Islamic cultural center near Columbus, Ohio, the secretary of homeland security, Jeh C. Johnson, gave no details when asked about his agency’s warning or the possibility that a terrorist plot was being hatched.
“We’re vigilant about the potential for domestic-based violent extremism on an ongoing basis,” Mr. Johnson said.
Osama bin Laden’s death in 2011, together with the rise of Qaeda affiliates in Yemen, Syria and elsewhere, led some analysts to conclude that Mr. Zawahri, Bin Laden’s successor, no longer had clout in the militant world.
In an age when savvy militants spread their messages using social media, Mr. Zawahri rarely produces audio or video recordings of his pronouncements. Obama administration officials for years have boasted that the C.I.A.’s campaign of drone strikes in Pakistan has devastated Al Qaeda’s apparatus there, but the emergence of the Khorasan Group in Syria appears to indicate that Mr. Zawahri’s authority and influence, however symbolic, endure in some corners of the universe of militant organizations.
Besides Mr. Fadhli, who was once one of Bin Laden’s close advisers and who according to the United Nations once fought against the Russian government in Chechnya, another top member of the Khorasan Group is believed to be Abdul Mohsen Abdullah Ibrahim al-Sharikh, a Saudi who also arrived in Syria in 2013. In August, the Treasury Department imposed sanctions on Mr. Sharikh, describing him as one of the Nusra Front’s “top strategists.”
Mr. Sharikh and Mr. Fadhli were once part of a cadre of Qaeda operatives living in Iran, facilitating the flow of money, weapons and fighters that moved from Iran, Afghanistan and Pakistan to Iraq. Many senior Qaeda operatives fled Afghanistan to Iran after the American war in Afghanistan began in 2001, and the exact circumstances of the Qaeda group in Iran have been one of the mysteries of the post-Sept. 11 period.
Iran’s government said the militants were living under house arrest, and American intelligence agencies do not believe that Iran — a Shiite-majority country — ever considered an alliance with a Sunni terrorist network. Starting at the end of the last decade, the Qaeda operatives began leaving Iran for Pakistan, Afghanistan and elsewhere. One of them, Bin Laden’s son Saad, was killed in a C.I.A. drone strike in Pakistan in 2009.
Others went farther afield — to the war in Syria that erupted in 2011 and that has since become the epicenter of the jihadist world.
Michael S. Schmidt and Eric Schmitt contributed reporting.