Aspen, Colorado CNN  — 

FBI Director James Comey says U.S. military strikes have diminished the al Qaeda offshoot Khorasan Group, but the bigger threat faced by the U.S. is now ISIS.

Comey, in an interview with CNN’s Wolf Blitzer, used an appearance at the Aspen Security Forum to raise concerns about encrypted communications the FBI can’t access, comparing ISIS militants to needles in a regional haystack.

“If you imagine a nationwide haystack, we are trying to find needles in that haystack. And a lot of those needles are invisible to us either because in they are communicated or just because they have communicated in a place that we can’t see them,” Comey said. “And knowing there are needles out there that you can’t see is very worrisome.”

Comey said Wednesday SIS has become a bigger threat to the United States than al Qaeda.

“The threat that ISIL presents and poses to the United States is very different in kind, in type and degree than al Qaeda,” Comey said. “ISIL is not your parents al Qaeda. It’s a very different model. And by virtue of that model, it’s currently the threat we are worried about in the homeland most of all.”

Also at the security forum was Department of Homeland Security Secretary Jeh Johnson, who said the imminent threat comes from the potential smaller-scale attacks within the United States, giving the recent shooting in Chattanooga as an example.

“Abdulazeez was not on our radar and I wouldn’t consider Chattanooga a high risk area,” Johnson said.

Johnson said the long-term effects could come from concerns that the terror organization, ISIS, will use the large portion of land it has obtained to train fighters and then send them to the West.

What is the Khorasan Group?

The Pentagon announced Tuesday Muhsin al Fadhli, a Kuwaiti-born jihadi and leader of the Khorasan Group, was killed earlier this month in a targeted strike. The strike happened July 8 while Fadhli was traveling to Sarmada. Syria.

“His death will degrade and disrupt ongoing external operations of al-Qaeda against the United States and our allies and partners,” Pentagon spokesman Capt. Jeff Davis said in a written statement.

Why ISIS is winning and how to stop it

While terrorist groups like ISIS and the al Qaeda affiliate Jabhat al-Nusra are responsible for much of the violence inside Syria, the Khorasan Group was believed to direct most of its energy plotting external attacks in the West.

However, Johnson also defended the Obama administration’s decision to not call terrorism “Islamic extremism.”

“I think ISIL believes what it’s doing is driven by religion. The Muslims I know in this country believe just the opposite. This band of criminals does not represent the overwhelming majority believes is what Islam is all about,” Johnson said.

Comey also said Wednesday that investigators haven’t determined why Mohammad Abdulazeez carried out the shootings that killed four Marines and a sailor last week in Chattanooga, Tennessee. He said the FBI is determined to “understand every second of his life” for the last two years, at least.

Comey said the prospect of a terrorist group launching a cyberattack on the United States is a small but growing problem.

Obama on ISIS: ‘This is a long-term campaign’

CNN’s Sophie Tatum contributed to this report.