Captured ISIS fighters provide clues to remains of Jihadi John victims

The two men have given Kurdish Syrian Democratic Forces details about potential burial locations of several US and Western hostages killed by Mohammed Emwazi, the British ISIS operative known as Jihadi John responsible for the beheading of several American hostages, the officials said.

The ISIS fighters have been identified by US intelligence as Alexanda Kotey and El Shafee Elsheikh, known associates of Jihadi John.

The New York Times was the first to report the capture of the two men.

Emwazi was killed in a US airstrike in Syria in 2015.

It has long been believed the hostages he beheaded in a series of violent videos were killed outside Raqqa. US intelligence has long suspected a general location, but had not been able to send US personnel on a search without more specific details and because there has been fighting in the area until recently.

US personnel will have to recover the remains and then confirm any potential identities through DNA testing.

The US is informing the families of Americans and others held and killed in Syria there are new leads on the possible whereabouts of the remains of their loved ones.

But the officials have cautioned it is very early in the process. The remains still have to be fully recovered and undergo an identification process the officials say.

Kurdish forces, mainly Syrian Democratic Forces, are holding 'hundreds' of detainees, says one official. Interrogations of those with the most valuable information are also yielding fresh intelligence to US special forces involved in analyzing information about the current state of ISIS' capabilities and planning.

One US assessment is that thousands of ISIS operatives have escaped Syria in recent months and are potentially back in Europe, potentially plotting new attacks, one US official with direct knowledge of the latest information tells CNN.

The assessment includes growing concerns these one-time foreign fighters are now able to plan fresh attacks in the West officials say.

In particular, there is concern that the Turkish Syrian border has again become porous in recent weeks since Turkish forces began conducting their incursion into northern Syria.

There is also intelligence that shows a significant number of fighters are crossing back into Turkey and then moving into Libya and Egypt.

Fighters have been caught with sophisticated forged documents made by ISIS, contributing to a continuing concern that the terrorist group remains far from down and out, one official said.