Story highlights Khorasan Group has been thought a target of U.S. airstrikes

After first strikes in Syria, reports said top Khorasan members had been hit

Official now says "its 99.5% certain" they are alive

The U.S. intelligence community now believes two key terrorist operatives targeted by the United States in the opening night of attacks in Syria are still alive and could be actively plotting, multiple officials tell CNN.

The operatives are key members of Khorasan Group, the al Qaeda affiliate entrenched in Syria that the United States has declared poses a great risk to American national security. One official with direct knowledge of the latest U.S. assessment said the working assumption now is that both Muhsin al-Fadhli, the leader of the group, and David Drugeon, a French jihadist and key member, who is believed to be a skilled bomb-maker, are alive. The United States does not know with certainty if they are injured.

An intelligence analyst with knowledge of the intelligence tells CNN "its 99.5% certain" they are alive.

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There had been scattered press reports about the fate of both men. But until now the United States had not indicated this strongly that it believes both men survived or left before a barrage of 47 U.S. Navy Tomahawk missiles on September 22 on several suspected Khorasan sites in Syria. Officials said news reports on Khorasan Group before the strikes may have had an impact on the effectiveness of striking the group.

Drugeon is believed to be heavily involved in facilitating the movement of fighters back and forth from Europe, and in planning attacks in Europe. His name has not been widely disclosed by the United States. Like al-Fadhli and the rest of the Khorasan Group, U.S. officials say Drugeon has ties to the core al Qaeda group in Pakistan and is believed to have come to Syria from Pakistan in the last two years.

U.S. officials said he is believed to be one of the key bomb-makers in the group and may have been actively involved with creating easily concealed bombs that led to increased security measures at overseas airports this past summer. U.S. intelligence officials said it's possible he may still be living in Syria. The United States is tracking a number of al Qaeda leaders believed to have moved into Syria, some having transited Iran to get there.

Al Qaeda in Yemen's master bomb-maker, Ibrahim Al Asiri, is believed to have shared techniques and technology with Khorasan. One of the targets during the U.S. attack this fall was a bomb-making facility.

Another U.S. official told CNN about a month ago there were communications intercepts suggesting the militants were discussing the possibility of Al Asiri traveling to Aleppo, Syria, to offer direct assistance to Khorasan and the Al Nusra Front, a related jihadist group.

U.S. officials believe Al Asiri did not take the risk of traveling to Syria and would not do so because of the presence of U.S. reconnaissance and surveillance over Syria and the stepped up airstrikes. It is not clear to what extent those intercepts were verified as representing Al Asiri's views.

U.S. officials have said recently they believe the missile attacks against Khorasan did not destroy the group and that it still poses an imminent threat to the United States because of its ability to make bombs that are not readily detected by airport screening technology.

The recently retired director of the National Counterterrorism Center, Matthew Olsen, said the threat from the group is still significant.

"This group was in a position to train without any sort of interference, they were able to recruit operatives. We saw that they were looking to test explosives. So they were in the advanced stages of plotting. And again they had both intent and that capability that put them nearing an execution phase of an attack," Olsen told CNN"s Jim Sciutto.

"I don't think there's any realistic likelihood that some limited airstrikes even just for a period of time will degrade that threat altogether. Those individuals, they're hardened, seasoned veterans, and they've got an ability to operate pretty freely in Syria. So I think it's unlikely that threat's altogether been eliminated."

The United States has not acknowledged striking against the Khorasan Group since that first night in September.