The Pentagon confirmed Tuesday that it had killed the leader of an Al Qaeda offshoot tasked with planning attacks on the West, putting in the spotlight a group that remains among the most mysterious of the world’s jihadist groups.

Muhsin al-Fadhli, the leader of the Khorasan group, was killed in an airstrike in Syria on July 8, according to Pentagon spokesman Capt. Jeff Davis. The group is believed to be embedded with the Al Nusra Front, Al Qaeda’s affiliate in Syria.

Signaling Mr. Fadhil’s importance, Captain Davis said that he was one of the only Al Qaeda members to know about the 9/11 attacks in advance. The US issued a $7 million reward for information leading to his capture or death.

The Associated Press reports:

Officials have said the Khorasan militants were sent to Syria by the al-Qaida leader Ayman al-Zawahiri to recruit Europeans and Americans whose passports allow them to board a US-bound airliner with less scrutiny.

According to classified US intelligence assessments, the Khorasan militants have been working with bomb-makers from al-Qaida’s Yemen affiliate to test new ways to slip explosives past airport security.

Officials were worried they would provide these sophisticated explosives to their western recruits who could sneak them on to US-bound flights.

Although the Islamic State has garnered far more public attention in the last year than Al Qaeda, the Khorasan Group has remained a key focus for the US. The small cadre of senior operatives is considered one of the greatest threats to the US.

The lack of information about the group makes it difficult to draw conclusions about the implications of Fadhli’s death. But although Al Qaeda and its affiliates have proven resilient, surviving despite the death of several key leaders, analysts say that Fadhli’s death is a significant blow.

“While the threat they pose will persist, the loss of Muhsin’s leadership and experience is a real setback for the group,” Matthew Olsen, a former director of the National Counterterrorism Center, told The New York Times.

The Times reports that Khorasan is believed to have about two dozen members from across the Middle East, South Asia, and North Africa. They were reportedly sent to Syria by Al Qaeda leader Ayman al-Zawahiri.

The US first mentioned the group publicly in September 2014. The timing of the announcement, just ahead of the beginning of US air strikes in Syria, raised suspicions about the severity of the threat posed by the group.  Firstlook published a story accusing the US of fabricating the group entirely.

In May, Al Nusra leader Abu Mohammed al-Jolani denied the existence of the Khorasan Group, telling Al Jazeera that “there is nothing called Khorasan group. We heard this from the Americans only.”

The Carnegie Endowment for International Peace produced a detailed profile last September. In it, author Aron Lund suggests that the idea was for Khorasan group to carry out attacks that could be claimed by Al Qaeda’s central leadership, which has been losing legitimacy amid the very high-profile victories of the Islamic State.

Mr. Lund writes that this model – an “external operations division within, or on the fringes of” another Al Qaeda-affiliated group was first practiced in Yemen, within Al Qaeda in the Arabian Peninsula.

The group was responsible for a string of failed bombings against U.S. aviation in 2009 and 2010.

This sudden spike of attacks coming out of far-off Yemen was no coincidence. It appears to have been the result of a decision by al-Qaeda and AQAP in the late 2000s to form an external operations group within AQAP, tasked with the preparation and organization of attacks against high-priority targets outside Yemen. This decision was accompanied by new efforts at recruiting Westerners and inciting “lone wolf” attacks in Europe and the United States, for example by the publication of Inspire, an English-language propaganda newsletter that began online publication in mid-2010.

The US has not released any information about attacks being planned by the Khorasan group at the time of Fadhili’s death.