Who is the leader of ISIS?

'Greta Investigates: ISIS': Abu Bakr al-Baghdadi has proven to be enigmatic and elusive, but how did he rise to power?

This is a rush transcript from "On the Record," September 19, 2014. This copy may not be in its final form and may be updated.

GRETA VAN SUSTEREN, FOX NEWS HOST: So who is the leader of ISIS? Here's what we know about the allusive and vicious Al-Baghdadi.

(BEGIN VIDEOTAPE)

ABU BAKR AL-BAGHDADI: (SPEAKING FOREIGN LANGUAGE)

VAN SUSTEREN: Little is known about America's most wanted terrorist, 43-year-old Abu al-Baghdadi. The ISIS leader has a $10 million U.S. bounty on his head. In July, he made a rare public appearance at a mosque in Mosul, Iraq, wearing what appeared to be an expensive watch. He cleaned his teeth before speaking to a captive audience. The self-proclaimed leader of ISIS went on a 15-minute rant.

AL BAGHDADI: (SPEAKING FOREIGN LANGUAGE)

TIMOTHY FURNISH, PROFESSOR OF ISLAMIC STUDIES: Abu Bakr al-Baghdadi has proclaimed himself in two different terms, the Khalifa, which means successor to Muhammad, and as political and military leader.

VAN SUSTEREN: When video of Baghdadi's sermon was posted online, it was the first time many of his followers had ever laid eyes on him.

AL BAGHDADI: (SPEAKING FOREIGN LANGUAGE)

FURNISH: Al-Baghdadi was also known as Ibrahim al Awad. Sometimes also affixed to that is al Samari, which means that part of Iraq. He is an Islamic scholar. There's some Western scholars that have sort of ridiculed his credentials.

KEVIN CARROLL, FORMER U.S. ARMY INTELLIGENCE OFFICER & CIA CASE OFFICER IN MIDDLE EAST: Abu Bakr al-Baghdadi has been a shadowy presence for a very long time.

VAN SUSTEREN: As a U.S. Army Intelligence Officer, Kevin Carroll served three tours in Iraq and Afghanistan. After his military service, he was a CIA case officer in the Middle East.

CARROLL: There was questions as to whether or not he really existed. He was sometimes confused with another high-value target who led al Qaeda in Iraq who was killed.

(GUNFIRE)

CARROLL: He may have just been in the battle of Fallujah when the Marines reduced the al Qaeda hamlet in Fallujah in 2004, 2005. He was reportedly in U.S. custody at Camp Bucca in Iraq from 2005 to 2009.

VAN SUSTEREN: The now-closed prison camp located in southeastern Iraq was named after Ronald Bucca, a New York City fire marshal killed on 9/11.

CARROLL: Consistent with the U.S. drawdown of forces and the turnover of authority to the Iraqis, he was improperly let go out of custody in 2009.

VAN SUSTEREN: Details are sketchy about what happened next.

CARROLL: We do know that al Qaeda had the great amount of autonomy within those camps in Iraq. They still kept their command structure. They were still ordering and planning attacks from inside custody.

VAN SUSTEREN: In the years following his release, Baghdadi quickly climbed the ranks of al Qaeda in Iraq. Later, the group rebranded itself as ISIS.

FURNISH: He clearly sort of leapfrogged ahead of al Qaeda by going ahead and declaring a territorial state, something that has, in many ways, empowered ISIS, the Islamic State to really now lay claim to being the chief Islamic terrorist organization in the world.

(SHOUTING)

(END VIDEOTAPE)

VAN SUSTEREN: Back with Colonel Oliver North. And joining us is Middle East expert and columnist, Lisa Daftari, and terrorism expert, Walid Phares, author of "Future Jihad."

Ollie, where is Baghdadi?

NORTH: He's probably in Raqqa, Syria. It's their headquarters, the place from which they've branched out and taken from the Levant to south of Baghdad as of yesterday. Very well-defended. We expect -- I think they have got the same kind of, if you will, physical infrastructure to protect them that we saw recently in Gaza.

VAN SUSTEREN: Lisa, you agree? I saw you nodding your head when Ollie spoke that he's probably there.

LISA DAFTARI, COLUMNIST & MIDDLE EAST EXPERT: Yeah, they know. And they built their infrastructure in Raqqa. That's when the U.S. wasn't looking and wasn't paying attention. The president was so fixated on his red line about Assad using chemical weapons and that's the time when Baghdadi jumped over from Iraq to Syria and they built the headquarters there.

And that's when they really began their plans on this caliphate, on Islamic State. Taking over oil fields. That's where they are getting their revenue. Taking over airfields and all the elements that are needed to make a state. And at the same time, doing public crucifixions. We have covered that here at FOX and we have been reporting about it all along. So building this caliphate and building their headquarters, he must be there because he has the protection.

But at times, he does show his face to invigorate the cause and to radicalize others to join them as well.

VAN SUSTEREN: Walid, I guess that's why the Syria part of this is so very important. The president doesn't talk as much about Syria. And the air strikes right now are not in Syria. But if he is hiding there and if he is the leader --

WALID PHARES, MIDDLE EAST AND TERRORISM EXPERT & AUTHOR: Let me say one thing. The whole theories are about him being in Raqqa, Raqqa definitely is the capital. Raqqa is his office and headquarters but he personally may not be there. Why? They have learned from what we have done with al Awlaki in Yemen with a drone. They have learned what we have done with killing bin Laden if we know he is in a fixed position. And he has to communicate. Unlike with bin Laden who had cut off everything, this man is the chief of staff and the commander of forces. There are communications, which means that while officially he could be there, he could be well mobile.

VAN SUSTEREN: Any idea where is he? No idea?

PHARES: All the area between Mosul and Raqqa is the area he could be moving in.

NORTH: What you just highlighted is one of the extraordinary problems we have. We have become so dependent on signals intelligence we have almost no human intelligence on the ground inside Syria to give us any reports on where he really might be.