Authorities believe Atar helped to radicalize at least one of the brothers online, according to a police report into the case seen by CNN.
Atar, 32, who has dual Belgian and Moroccan nationality, is suspected of having directed the attacks from Syria. He remains at large.
In September, Belgian authorities were worried Atar was hiding somewhere in Western Europe and described him as "the highest ranking ISIS operative" to have emerged from the outskirts of Brussels.
"That kind of individual comes to Western Europe for a reason," a judicial source said, noting that as things get more desperate for ISIS in Syria and Iraq, they've been sending "a lot" of people back to Europe to launch attacks.
The source also said they had reason to believe Atar was "most probably" in Brussels in August. Belgian Special Forces had been conducting raids in various neighborhoods when word leaked to the media. "It was a pity" the senior judicial source said. The leak had "immediate consequences" for the investigation.
Suicide bomber brothers
The source said investigators found proof of Atar's connection to the Paris and Brussels attacks on the laptop of one of the Bakraoui brothers
Ibrahim and Khalid El Bakraoui both detonated suicide bombs in the March 22 attacks in the Belgian capital that killed 32 people and injured more than 300
Ibrahim, along with another bomber, blew himself up in the departure lounge of Brussels Airport in Zaventem before his brother detonated his on the Brussels subway about half an hour later.
A few days after the Brussels attacks, Atar's brother, Yassine, was arrested on suspicion of terrorist-related offenses. Authorities say Yassine was found with a key to the apartment the Bakraoui brothers used to prepare for the Brussels attacks.
The attacks in Belgium came four months after an even deadlier assault on Paris on November 13, 2015
. Seven attackers killed 130 people in multiple gun and bomb attacks
across the French capital.
Investigators have long said the attacks were planned
and carried out by the same ISIS network. Two suspects in the attacks are in French custody.
Mohamed Abrini, who was seen in a surveillance video from the Brussels Airport with the bombers and has been linked to the Paris attacks through surveillance footage and DNA, was arrested in April.
Abrini has told Belgian investigators that he traveled to Raqqa
, the Syrian city that is the capital of ISIS' so-called caliphate, in 2015.
There, he met Abdelhamid Abaaoud
-- the man who organized the Paris attacks, and was later killed in a shootout with police.
Belgium has since handed over Abrini to French authorities.
is one of the 10 men accused of carrying out the Paris attacks. After four months on the run, he was arrested in a Brussels suburb in March, and has been extradited to France to await trial.
He and Abrini were identified together on surveillance video at a gas station in France two days before the Paris attacks.
According to Belgian network VTM, Abrini has suggested to investigators that Abdeslam was a key figure in the Paris plot who picked up most of the attackers coming from Syria and dropped them off at hiding places.
Atar an 'ISIS original'
Oussama Atar is described by authorities as an "ISIS original." He was arrested in Iraq in 2005. According to an Iraqi judgement obtained by CNN, an Iraqi tribunal convicted Atar in 2007 for crossing into the country illegally.
According to his lawyer, he was incarcerated in several prisons managed by the U.S. authorities: Abu Ghraib, Camp Cropper and Camp Bucca. Belgian authorities tell CNN that's where he may have met the future leader of ISIS, Abu Bakr Al Baghdadi.
In 2010, Atar complained about his health to Belgian government representatives in Iraq.
"His mom called and she was worried. She said that the Belgian Foreign Ministry told her that he was about to die of kidney cancer if he didn't get surgery," Vincent Lurquin, Atar's former lawyer told CNN.
Back in Belgium, his lawyer, family, various human rights organizations and members of parliament launched a public campaign, calling for his transfer to a health care facility.
Jailed in Iraq
"Oussama Atar, a 26-year-old Belgian national is in need of urgent medical care at al-Rusafa Prison in Iraq," Amnesty International wrote in a press release.
"According to information provided to the Belgian embassy by the prison authorities he has a tumour in the kidney and must undergo an urgent operation in the next two weeks. Reports also indicate he has lost a great amount of weight and is very ill. Amnesty International fears for his health and is calling on the Iraqi authorities to provide urgently needed medical care"
Eventually Atar was treated by doctors from the Red Cross. "That's when we realized that he didn't have cancer, but a colon infection," Lurquin told CNN.
When asked about this campaign, Amnesty International wrote another statement saying that "access to necessary medical care is a basic human right and yes, even prisoners have rights," and that "their action wasn't for his release and this was not our goal."
In September 2012, Atar was released from prison and sent back to Belgium. Upon arrival he called his lawyer to assist during the procedural interrogation.
"Investigators didn't ask him a lot of questions about what happened there. At that time nobody was really speaking about ISIS in Belgium. After he got released nobody was really interested in Ossama. They left him by himself." Lurquin told CNN.