UPDATED 03/25/16 12:07 a.m.

BRUSSELS (CBSNewYork/AP) — Belgian authorities made six arrests Thursday in connection with the Brussels terrorist attacks, but were still searching for at least two suspected ringleaders.

Among them was the third man who escaped after the terrorist bombing in the Brussels Airport at Zaventem this week, a U.S. law enforcement source told CBS News.

Belgian authorities continued to search for the third unidentified man at the airport wearing the white jacket and hat who fled (pictured below).

Suicide bombers, suspect in Brussels attacks (Belgian police)

Suicide bombers, suspect in Brussels attacks (Belgian police)

Sources said immediately after the airport bombing, he was believed to have boarded a shuttle to another area of the airport and has not been seen since.

Meanwhile Thursday, Belgian state broadcaster RTBF and France’s Le Monde were reporting that a second attacker is suspected of taking part in the bombing of a subway train just after the airport attack.

Belgian media published a sketch of the man who was filmed by surveillance cameras in the Brussels metro on Tuesday carrying a large bag alongside Khalid El Bakraoui, the alleged suicide bomber in the metro attack.

The attacks killed at least 31 people and injured more than 200.

BRUSSELS ATTACKS: Photos | Videos | Coverage From CBS News

Two other men captured by surveillance cameras at the airport were believed to be suicide bombers. Officials confirmed Wednesday one of them was Belgian national Najim Laachraoui. He is also believed to be the bombmaker for the Paris attacks back on Nov. 13 of last year, CBS2’s Alice Gainer reported.

Belgian officials have not publicly linked any of the remains to Laachraoui; nor have they said he was involved in the Brussels attack.

But two security officials told the Associated Press that Laachraoui’s DNA was verified as that of one of Tuesday’s suicide bombers after samples were taken from remains found at the airport.

According to Belgian authorities, Laachraoui’s DNA was also found on the explosives used in the gun and suicide attacks in Paris, CBS News’ Charlie D’Agata reported.

Belgian authorities have been looking for Laachraoui since last week, suspecting him of being an accomplice to Salah Abdeslam, who was arrested Friday in the Brussels neighborhood where he grew up after four months on the run following the Paris attacks.

CBS News has learned the attacks were originally planned for Monday, March 28, but were moved up after police captured Abdeslam.

The other suicide bomber in the airport surveillance photo was identified as Ibrahim El Bakraoui – the brother of Khalid El Bakraoui.

The Bakraoui brothers have also now been linked to a plan to target a nuclear power plant in Belgium, CBS News’ Kenneth Craig reported. They were also on a U.S. terror watch list.

Interpol issued a red-level arrest warrant for Khalid El Bakhouri in December, when Belgian police connected him to one of the hideout apartments used to plan the November Paris attacks, CBS2’s Jessica Schneider reported.

Ibrahim El Bakhouri was caught near the border between Turkey and Syria and was deported to the Netherlands in 2015 and eventually got back to Brussels. Turkey warned Dutch and Belgian officials that he was a foreign terrorist fighter.

Late Thursday, police were still swarming the streets in Brussels and searching apartments.

The attack in Brussels and the events in Paris are renewing concerns about battling ISIS in Europe.

“It’s much easier to get into Europe than to get into the United States for example because of geography,” said Daniel Benjamin, former U.S. Ambassador at Large for Counterterrorism. “Most European Muslims are not radicalized, but they do have a higher radicalization rate.”

In a second claim of responsibility Wednesday, the Islamic State group warned of further attacks and what it called “dark days” for countries involved in attacking IS positions in Syria and Iraq.

According to the Associated Press, ISIS has trained at least 400 fighters to carry out waves of attacks in Europe designed to inflict maximum carnage.

Speaking from Argentina, President Barack Obama once again urged the international community to come together in the terror fight.

“We can and we will defeat those who threaten the safety and security of not only our own people but people all over the world,” he said.

Meanwhile, Abdeslam’s lawyer, Sven Mary, said his client is not fighting extradition to France, which is seeking his extradition from Belgium to face potential terrorism charges.

Mary told reporters in Brussels on Thursday that he asked for a one-month delay on any transfer while he studies the large dossier.

He said that Abdeslam “wants to leave for France as quickly as possible.”

Meantime, survivors of the Brussels attack were speaking Thursday.

Sebastien Bellin was at the Brussels Airport at Zaventem and remembers turning a corner in the terminal when the first bomb went off. He fell when the explosion happened.

“I looked down and you know, I see massive bones just sticking out. And then I hear the second explosion,” Bellin said. “And now that I’m starting to think about it over and over, I think that’s really what saved me, because I was already on the ground when the explosion – the second one went off.”

Another man who survived the attack in the metro system said he feels anger, sadness, powerlessness, and fear – and recognizes that he is alive and lucky.

The bombing was also believed to have killed a brother and sister from New York.

Sascha and Alexander Pinczowski have family in the Netherlands, but had been living in Manhattan.

Many have been posting about their disappearance on social media.

They were reportedly on the phone with their mother, when she heard an explosion and the phone went dead.

Their parents said late Thursday that they were not among those listed at any Brussels hospitals.

Wyckoff, New Jersey native Ashley Bruggemann was in Brussels for work at the time of the attacks, and said her decision to get to the airport 30 minutes early saved her – since she was already well past security when the explosions sounded.

“I thanked every Guardian angel that I had that kept me safe that morning,” Bruggemann said.

Also Thursday, two Belgian officials tried to resign over intelligence failures leading up to the attacks, but were rejected.

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