Tuesday's explosions in Brussels will raise new questions about Islamist extremism in Belgium — a country which experts say has the highest number per capita of militants fighting in Syria and Iraq. In December, NBC News investigated Belgian-born Paris attacks ringleader Abdelhamid Abaaoud.

ISTANBUL — Videos and photographs belonging to the ringleader behind the Paris terror attacks offer a glimpse of his life as a "gore hero" on ISIS' front lines.

Anti-ISIS activists told NBC News they obtained the files from Belgian-born Abdelhamid Abaaoud’s cellphone in northern Syria in early 2014.

The series of nearly 50 digital photographs and video clips show shocking scenes of death and brutality, but also commonplace images of people in unremarkable settings. There are also pictures of rental car flyers and one of a boat.

Abaaoud, 27, died in a gunbattle with police five days after a series of shootings and bombings in the French capital killed 130 people. In addition to participating in assaults on a cafe and restaurant in Paris, officials allege Abaaoud was involved in four of the six attacks foiled by French intelligence since the spring.

Paris terror attacks ringleader Abdelhamid Abaaoud in a photo obtained by anti-ISIS activists from his cellphone in northern Syria in early 2014. NBC News

One of the most haunting video clips in the collection shows Abaaoud and his fellow militants examining the aftermath of a massacre.

“Look at his face! His face has been crushed,” one of the militants says in North African-accented Arabic off camera, as Abaaoud films a close-up of a corpse. “He was fighting for democracy, for secularism. He was fighting for money.”

It is not clear exactly when or where the video was shot, although the clothing and setting suggest the dead men are rival rebel fighters in Syria. In the clip, Abaaoud says that one unarmed dead man appeared to have Down syndrome.

The files also include:

A photo of a topless woman whose breasts are concealed by a motorcycle

A photo showing a pile of bodies behind a Dodge 4x4 pickup truck

A photo showing a shelf containing of lotions that appears to have been taken in a spa

A meme of a woman whose face is covered by a niqab with text stating in French: "If a woman is free to show her body, does she not also have the freedom to cover it?"

A video showing a beheaded body that appears to have been taken in the Syrian town of Azaz

A photo of a man wearing sunglasses leaning against a silver Mercedes-Benz.

Abaaoud, the son of Moroccan immigrants, was raised in the poor Molenbeek neighborhood of Brussels — an area with a long history of links to extremism.

He had featured prominently in ISIS propaganda, and his ties to other terror cells that had successfully carried out attacks on European soil put him on the radar of authorities across the continent. Many of the cellphone images of Abaaoud from the files ran on Belgian and French TV earlier this year after a deadly January anti-terror raid in Verviers, Belgium.

Despite his notoriety, Abaaoud was still able to take advantage of borderless travel between 26 European countries and moved about freely — once even boasting about evading Western intelligence agencies.

A photo of a car rental ad found on a cellphone belonging to Abdelhamid Abaaoud. NBC News

“Not only was Abaaoud known for threatening attacks, but some of the images of Abaaoud had been widely circulated. His face was not even concealed,” said Gilles Kepel, a professor at the Paris Institute of Political Studies and an expert in European jihadism. “He was a sort of gore hero of the jihad, if you wish, playing with corpses and everything.”

For the Syrian activists who shared the files with NBC News, it was only in the aftermath of the attacks in Paris last month that they realized the full significance of what they had collected.

“It was a total coincidence,” Ali, one of the activists, said in an interview in Turkey. (NBC News has agreed not to disclose Ali’s real name.) “When the [Nov. 13] attacks happened, and then some of the pictures were released about the ringleader of the Paris attacks, we realized that we have some of these videos on file [from] more than a year ago.”

Abdelhamid Abaaoud in another of the photos from his cellphone obtained by anti-ISIS activists. NBC News

The activists previously had shown the files to Belgian journalists researching local militants earlier this year.

Tiny Belgium — with a population of just 11 million — has the highest number per capita of militants fighting in Syria and Iraq, experts say.

Pieter Van Ostaeyen, a Brussels-based researcher and analyst, said that at least 500 Belgian nationals among of the thousands of European nationals who are fighting for militant groups in Syria.

“It’s impossible to say who comes back and at what time. They don’t go posting that on Facebook, like ‘Hey! I’m coming back to Europe. Prepare yourselves!’” Van Ostaeyen said. “If they come back with bad intentions, we’ll only know as soon as they attack us.”

Ayman Mohyeldin and Tim Uehlinger reported from Istanbul and Mac William Bishop reported from Brussels and Paris. Chloe Consigny reported from Paris.