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Brussels attack investigation: Brothers were suicide bombers in two locales

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(CNN) -- Several bombers are dead. At least one alleged attacker is still on the loose.

And a key question looms as investigators race to piece together details about the attackers behind Tuesday's deadly bombings in Belgium's capital: Were these men acting alone, or were other members of a terror cell supporting them?

Two of the bombers were brothers, officials said.

Belgian federal prosecutor Frederic Van Leeuw identified Ibrahim El Bakraoui as one of two suicide bombers at the Brussels airport and his brother, Khalid El Bakraoui, as the man behind a deadly suicide blast about an hour later on a train near the Maelbeek metro station.

And one of the bombers at the airport may be the man authorities named as a suspect in the Paris terror attacks.

Belgian investigators believe that the second suicide bomber killed in the Brussels airport was alleged bomb-maker Najim Laachraoui, multiple European officials told CNN. They have not established his identity conclusively, a senior Belgian counterterrorism official told CNN's Paul Cruickshank, and are checking against DNA and fingerprint records.

Earlier this week, police identified Laachraoui, 24, as a suspect in the November Paris attacks and said they were intensifying their search for him.

Raids, arrests and forensic analysis are some of the tools investigators are using to get to the bottom of the attacks, which killed 31 people and wounded 270 others.

The stakes are high, Belgian counterterrorism official Paul Van Tigchelt said Wednesday.

"There are still a number of people, possibly involved in the attacks still in our country ... who still pose a threat," he said.

Suspect on the loose

Surveillance images showing three men pushing luggage carts through the airport have played an important role as authorities work to pinpoint the suspects.

Authorities say Ibrahim El Bakraoui, who died in the bombing, is one of three attackers shown in the image.

They believe Laachraoui is one of the other men in the photo, but haven't said which one.

Authorities have said they're looking for the man show wearing light-colored clothing and a hat. Belgium's interior minister said that man placed a bomb at the airport and left.

While two explosives went off within 37 seconds of each other shortly before 8 a.m., this third bomb -- described as the "heaviest" by Van Leeuw -- did not, instead being detonated by authorities later in a controlled explosion.

Raids and arrests

Two people were arrested in Brussels in connection with the attacks -- one in Schaerbeek and the other in Haren, Van Leeuw said.

One was released later that day, according to the prosecutor.

Another person was detained Wednesday, according to Belgian state broadcaster RTBF.

One raid, officials said, came after a tip from a taxi driver led them to the northeast Brussels area of Schaerbeek.

The driver recognized the men shown in surveillance footage and told authorities he'd driven the men to the airport before the attacks. Police raided the area where the driver told them he'd picked up the men.

Investigators found chemical products and an ISIS flag during a house search in Schaerbeek, the federal prosecutor's office said in a statement.

On Wednesday, they made another significant find: Ibrahim El Bakraoui's will.

Police found the airport bomber's will on a computer in a trash can in Schaerbeek, Van Leeuw said.

The will indicated Bakraoui "needs to rush" and "no longer feels safe."

Links to Paris attacks?

The more authorities dig, the deeper the connections appear to be between the attacks in Brussels and those four months ago in Paris, where 130 people died in a terrorist massacre inside a concert hall, in cafes and restaurants and on the city's streets.

Investigators know that several of the Paris attackers had spent time in Belgium. One of them allegedly was Salah Abdeslam, who has been identified as the lone member of the core group of eight Paris attackers to survive.

Abdeslam was captured following a gunbattle with police Friday in Brussels. Belgian officials said that the 26-year-old may have been helping plan new attacks at the time of his capture.

Investigators believe Abdeslam likely planned to be part of an attack orchestrated by the same ISIS cell that carried out Tuesday's attacks, a senior Belgian counterterrorism official told CNN's Paul Cruickshank.

The Brussels attackers likely accelerated their plans when police discovered Abdeslam's hideout, investigators believe. The Belgium-born French citizen is thought to have eluded authorities -- while Mohammed Belkaid covered for him until being killed by a Belgian police sniper -- that first time, only to be caught a few days later.

That initial March 15 raid, in the southern Brussels district of Forest, was on a residence that one of the Bakraoui brothers rented using false identity papers, said a senior Belgian counterterrorism official.

A different Belgian security source said that Khalid El Bakraoui, specifically, rented that apartment and that he and his brother had ties to the November 13 carnage in Paris.

Attackers had criminal records

Interpol had a standing "red notice" for Khalid El Bakraoui, the subway bomber, that noted Belgian authorities wanted him in connection with terrorism.

And the Turkish presidency's office noted Wednesday that authorities captured Ibrahim El Bakraoui in June 2015 and flagged him to Belgian authorities shortly thereafter.

Speaking about the Bakraoui brothers, Van Leeuw said, "These two deceased suicide bombers had lengthy criminal records but (were) not linked to terrorism."

It's not clear how seriously Belgian officials took his case, though one expert thinks that having the Bakraouis' names public now should help with the investigation.

"You can start basically peeling back the onion," said Cedric Leighton, a CNN military analyst and the former deputy director for the Joint Chiefs of Staff. "Hopefully, what it will do is it will speed up the process by which they can actually look at all of the different elements of this and possibly roll up some more suspects."

Clues from explosives?

Another piece of evidence authorities found during the Brussels raids could help in their investigation: unused explosives.

In the Schaerbeek residence, authorities found 15 kilograms of the explosive TATP and screws among the bomb-making materials there, Van Leeuw said.

French prosecutors have said that the bombs used in the November Paris attacks -- which, like those in Brussels, ISIS claimed responsibility for -- were also made from TATP, which stands for triacetone triperoxide.

"Such bombs have been a signature of jihadist terrorists in the West for more than a decade because the materials are so easy to acquire, unlike military-grade explosives, which are tightly controlled in much of the West," CNN National Security Analyst Peter Bergen said.

TATP-based bombs require technical know-how and bulk purchases of hydrogen peroxide or hair bleach. That helps authorities narrow down potential bomb-making suspects, because making the explosives can sometimes bleach hair. So authorities can identify bomb-makers in part by recognizing unusually bleached hair or asking sellers to report any suspiciously large purchases of hydrogen peroxide.