- Authorities believe an ISIS bomb-maker was killed in the airport blast
- Investigators are trying to pinpoint who else could have been involved
- Turkish authorities caught Ibrahim El Bakraoui and flagged him to Belgium, Turkey's presidency says
(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?
Raids, arrests and forensic analysis are some of the tools investigators are using to get to the bottom of who was behind the attacks in Brussels, which killed 31 people and wounded 270 others.
Two of the bombers were brothers. And one of the bombers at the airport appears to be a man authorities named as a suspect in the Paris terror attacks.
But the investigation is far from finished. With at least one suspect on the run, 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.
Here's a look at the latest developments in the investigation, and the questions they raise.
Bombers were brothers
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.
This isn't the first time they've come across authorities' radar.
Ibrahim El Bakraoui was deported by Turkey to the Netherlands last year, a senior Turkish official told CNN. The Turkish presidency's office said authorities there captured him in July 2015 and flagged him to Belgian authorities.
Belgian authorities, the Turkish official said, responded soon after saying he had a criminal record but no known ties to terrorism.
Van Leeuw offered a similar assessment of the brothers Wednesday.
"These two deceased suicide bombers had lengthy criminal records," he said, "but (were) not linked to terrorism."
Ibrahim El Bakraoui had been sentenced in October 2010 by a Brussels criminal court to nine years behind bars for opening fire on police officers with a Kalashnikov during a robbery, according to Belgian public broadcaster RTBF and CNN affiliate RTL.
Interpol had issued a "red notice" for Khalid El Bakraoui, the subway bomber, that noted Belgian authorities wanted him in connection with terrorism. But it wasn't clear when that notice was issued or why Belgian authorities now say he had no ties to terrorism.
Question to consider: If they were already on authorities' radar, how did the brothers manage to slip through the cracks and carry out the deadly attacks?
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 bomber Ibrahim El Bakraoui is the man in the middle. Najim Laachraoui, an ISIS bomb-maker, is the man on the left in the picture, a Belgian counterterrorism official told CNN's Paul Cruickshank.
Investigators believe both were killed in the airport blast. But authorities are looking for the third man in the photo, walking on the right and 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.
Questions to consider: Where did the man in light-colored clothing go? Is anyone helping him hide out from authorities, and could he be plotting another attack?
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 public 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."
Questions to consider: Who are the people who were arrested, and what was their alleged role in the attacks?
Links to Paris attacks
The more authorities dig, the more connections they find between the coordinated attacks in Brussels and the string of shootings and bombings four months ago in Paris.
The latest connection: Laachraoui, a suspect in the Paris attacks who authorities now say they believe was one of the Brussels airport bombers.
Paris attacks suspect Salah Abdeslam, who authorities captured after a gunbattle last week, could be another link.
Belgian officials have said that the 26-year-old Belgium-born French national may have been helping plan new attacks at the time of his capture in a Brussels suburb.
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 Cruickshank.
The Brussels attackers likely accelerated their plans when police discovered Abdeslam's hideout, investigators believe.
And one of the apartments where he hid before his capture, located in the southern Brussels district of Forest, allegedly has ties to one of the Bakraoui brothers.
A Belgian security source said Khalid El Bakraoui rented the apartment.
Questions to consider: What role, if any, did Abeslam's arrest play in the Brussels attacks? Are there other links between the Paris and Brussels attacks?
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.
Questions to consider: Will the bomb-making materials lead investigators to any new suspects or help them dismantle a terror cell?