The latest European and U.S. estimates show as many as 1,900 of those foreign fighters are back in Europe after stints in Iraq and Syria, the official said. The estimate covers the last few years since intelligence services across the world began tracking these movements.
Most of the fighters returning -- but not all of them -- joined ISIS while in the terror group's self-declared caliphate and remain committed to the group. Others became disenchanted and fled back to Europe. It's not clear how many of those who returned are plotting new attacks, the official said.
"The threat stream is as high as it's ever been" in Europe, the official said, and the tracking of possible plots and threats across that continent has not diminished since the attacks last year in Paris and the holiday season.
There is also a "major risk" of ISIS operatives mixing in with refugees and trying to travel to Italy from North Africa and other parts of the Mediterranean, a second U.S. official told CNN. The Italians, this official said, are expressing their alarm about this issue to the United States and other European allies.
ISIS growing in Libya
There have been a number of arrests across Europe in recent months of suspected ISIS operatives, some who are said to be inspired by the organization and others who have been ordered to undertake attacks. A huge worry for the United States now is that the growing ISIS organization in Libya has the independence and authority to order attacks by operatives in Europe, without getting coordination or approval from ISIS leaders in Syria and Iraq.
Part of the problem is the lack of a stable and strong national government in Libya, where the United States believes there are somewhere between 2,000 and 3,000 ISIS fighters.
U.S. Secretary of State John Kerry has met with his counterparts from Italy and Qatar, as well as top NATO officials, to discuss how the United States could support Libyan efforts to form a new national government -- something Washington believes is vital, not just for the country's stability, but also to counter ISIS.
Many of the ISIS fighters in Libya are centered around Sirte, where a new ISIS video shows brutal executions by the terror group.
Military, diplomatic options being considered
The State Department and Pentagon are working on both diplomatic and military ideas for what the United States might be able to do against ISIS, but several defense officials say there are no formal military options yet. The Pentagon, however, is looking at what new legal authorities are needed so it can target and strike ISIS in Libya, according to two defense officials.
Europol says threat is most severe in a decade
The terror threat to Europe was recently underscored in a report from Europol, the European Union's law enforcement agency. That report said the threat is the most severe in more than a decade. The "so-called Islamic State has a willingness and a capability to carry out further attacks in Europe, and of course all the national authorities across Europe are working to prevent that from happening. We know that there is a capability," said Europol Director Rob Wainwright.
In addition, there is a renewed look at what maritime capabilities ISIS could try to develop to be able to attack commercial shipping, including cruise ships, in the Mediterranean, a NATO official told CNN.
The Europol report also made one of the few public references to the psychological state of those who join ISIS.
"A significant portion of foreign fighters have been diagnosed with mental problems prior to joining" ISIS, the report explained.
Some U.S. officials say more attention needs to be paid to ISIS' death cult mentality and the use of its apocalyptic vision to recruit fighters who will specifically go to their death for the terror group.
All of this comes as U.S. military and intelligence agencies are working on a fresh assessment of ISIS' overall strength in Syria and Iraq. Several officials tell CNN the initial indicators are that the overall number of fighters has dropped in those two countries, but there is no consensus on the data at this point.