The barrage of US cruise missiles last month aimed at an al-Qaeda cell in northern Syria killed just one or two key militants, according to American intelligence officials who say the group of veteran fighters is still believed to be plotting attacks against US and European targets.
The strikes on a compound near Aleppo did not deal a crippling blow to the Khorasan Group, officials said, partly because many important members had scattered amid news reports highlighting their activities. Among those who survived is a French-born jihadi who fought in Afghanistan with a military prowess that is of great concern to US intelligence officials now.
"The strikes were certainly effective in setting back the Khorasan Group, but no one thinks they were a permanent solution or a death blow to the threats that come from this cell," said Rep. Adam Schiff, a Democrat and a member of the House Intelligence Committee.
On Sept. 22, the US fired 46 cruise missiles at eight locations to target the group. At the same time, American airstrikes struck targets associated with the Islamic State group in Syria.
One of the US missiles went awry and killed a dozen civilians in the village of Kfar Derian, according to witnesses, who said the dead included Mohammed Abu Omar, an activist in the northern province of Idlib. The US military says it has not confirmed any civilian casualties.
The limited effectiveness of the attack on the Khorasan Group is partly the result of a hazy intelligence picture that also has bedeviled the air campaign against Islamic State targets in Syria and Iraq.
The US lacks the networks of bases, spies and ground-based technology it had in place during the height of the wars in Iraq and Afghanistan, officials say, or even the network of human sources it developed in Pakistan and Yemen.
The existence of the Khorasan Group became public only weeks before the airstrikes, but US officials had been tracking it for up to two years. Officials said the group has a few dozen al-Qaeda members, some of whom are long-sought militants of the fighting in Afghanistan and Pakistan. They are working closely with Syria's al-Qaeda-linked Nusra Front, officials said.
The several current and former US officials spoke on condition of anonymity about the group because they were not authorized to discuss classified information.
Khorasan is a historical reference to a region that included parts of Iraq and Afghanistan.
In public, US officials have offered seemingly contradictory assessments of the attacks on the Khorasan Group.
Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said the strikes disrupted the group's plotting, but he did not know for how long. FBI Director James Comey said he believed the plots had not been disrupted and that the group remains a threat to the US Other intelligence officials embraced Comey's view.
Unlike the Nusra Front, which is trying to topple Syrian President Bashar Assad, the Khorasan Group is focused chiefly on carrying out an attack against the West, officials say. The group is said to have been trying to recruit Europeans and Americans whose passports allow them to board a US-bound airliner with less scrutiny.
In addition, according to classified US intelligence assessments, the Khorasan militants have been trying to make or obtain explosives that can be slipped past airport security. Among their sources, officials said, has been al-Qaeda's Yemen affiliate, which has put bombs on airplanes, though the bombs failed to explode.
The fear is that the Khorasan militants will provide these sophisticated explosives to their Western recruits who could sneak them onto US-bound flights.
News stories last month, including a Sept. 13 report by The Associated Press that first disclosed the group's significance as a terrorist threat, led some members to flee before the US military had a chance to strike their known locations, US officials said.
One Khorasan leader, Muhsin al-Fadhli, has been eulogized on jihadi web sites, but American officials are not convinced that he is dead. They said they believe that another senior militant was killed, but have declined to name him.
A second Khorasan figure, a French militant named David Drugeon, is believed to be alive. Drugeon, who was born in the Brittany region and converted to Islam as a youth, spent time with al-Qaeda in the tribal areas of Pakistan before traveling to Syria, French officials say.
He was identified as a member of the Khorasan Group by two US officials who spoke on condition of anonymity to discuss classified information.