09/03/2014 04:52 EDT | Updated 11/03/2014 05:59 EST

Top U.S. counterterror official on possible ISIL attack: 'Lethal, but not 9-11'

WASHINGTON - Don't expect a wide-ranging, 9-11 style attack from ISIL against the West, says the U.S. government's top counter-terrorism expert.

Think 5-24 instead — as in the May 24 shooting this year that killed four people at a Jewish museum in Belgium, carried out by a French national of Algerian origin who spent time fighting in Syria.

The director of the U.S. National Counterterrorism Center, Matthew Olsen, offered that assessment Wednesday during a Washington presentation on the terrorist rebel group that has caught the West's attention.

He began his address with a description of the Belgium attack, and described it more than once as the likeliest model for any future attack by homegrown terrorists returning from battle with the Islamic State in Iraq and the Levant.

"A lone offender. Possibly acting on his own, possibly acting at ISIL's direction. But in any case, a smaller-scale type attack," Olsen said during his appearance at the Brookings Institution think-tank.

"Lethal, but nothing like a 9-11-style attack."

That set the tone for the basic theme of his speech: that ISIL is cause for concern, not mass hysteria.

His sober assessment came in an increasingly heated political climate, with the Obama administration drawing criticism for acting too slowly against the Islamist rebels who have now beheaded two American journalists on video.

Olsen offered several bits of optimism:

— There's no intelligence pointing to an imminent threat against the U.S., he said.

— There's no sign that fighters are getting organized within the U.S.: "There's no indication at this point of a cell of foreign fighters operating in the United States. Full stop."

— The international campaign of airstrikes and humanitarian aid in Iraq is working.

"ISIL is losing arms, it's losing equipment and it's losing territory.... The strikes have begun to sap ISIL's momentum," he said.

"As formidable as ISIL is as a group, it is not invincible."

Then, there was the bad news:

—ISIL is quick to adapt, he said. Its battle strategy comprises a mix of terrorist and traditional military tactics, and it's evolving: "ISIL has proven to be an effective fighting force. Its battlefield strategy is both complex, and adaptive."

—Terrorist fighters are now involved in 11 insurgencies around the world.

For its part, ISIL was conducting five suicide attacks per month in 2012. Last summer, it was 30-40 per month. Now the group is earning as much as US$1 million per day in illicit oil sales, smuggling and ransom, he said.

It has more than 10,000 fighters and controls an area bigger than the United Kingdom, Olsen said.

—Terrorists have learned from the Edward Snowden leaks, he said. They have moved to more secure communication platforms, begun using encryption software, and in some cases are now avoiding electronic communication altogether. Olsen said this has hurt U.S. intelligence.

Foreign policy is back at the forefront of the American political debate.

Poll data suggests U.S. public appetite for a Mideast battle is now ticking upward. A Washington Post column this week headlined, "President Obama's unnerving happy talk," blasted the commander-in-chief for not appearing concerned enough.

On the other hand, even within the more traditionally hawkish Republican party there are voices arguing that American interventionism is part of the problem — not the solution — in the Middle East.

For his part, Olsen said ISIL is still no al-Qaida.

The group that planned the Sept. 11, 2001 terrorist attacks remains the leader of the global jihadist movement — at least for now, he said.

"ISIL has captured our immediate focus," he said. "But it is only one of the myriad groups that pose a threat to us."