BRUSSELS, Belgium -- Five suspects.
So far that's how many people authorities have identified that were involved in Tuesdays terror attacks in Brussels, Belgium, that killed 31 people and injured 270.
Authorities now believe a second suspect was involved in the train bombing, a senior Belgian security source told CNN on Thursday.
The suspect was spotted holding a large bag on surveillance cameras at the station, according to Belgian public broadcaster RTFB. It's not clear if he was killed in the blast, RTBF said.
That brings the total number of suspects to five.
They know the names of two bombers, brothers Khalid and Ibrahim El Bakraoui; they know an ISIS bomb-maker, Najim Laachraoui, died at the airport; and they have an image -- albeit a grainy one -- of one suspect who they say is on the run.
And they've uncovered what appears to be a hideout, where they found a nail bomb and chemicals.
In the process of piecing together clues, time is of the essence.
It's a race to stop the next attack on European soil -- and it's one they very may well lose, some analysts say.
"I think Britain is to follow, with more attacks in France," CNN intelligence and security analyst Bob Baer said. "They're finding so many bombs in the more apartments that they take down."
"I think they're shocked by the amount of armament and the number of followers and how big the network was connected to Paris and now Brussels," he said.
As CNN International Diplomatic Editor Nic Robertson puts it: "The dam has broken."
Authorities are still trying to find out if the suspects were acting alone or if they were being helped by members of a terror cell.
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.
Unraveling that network may take some time, however.
Baer says that European authorities are late to the game when it comes to developing a network of sources inside the communities where terrorists thrive.
"These communities of North African origin are very tight-knit. It's very difficult for the European services to get inside of them -- they were ignored for so many years," Baer said. "Getting sources inside these groups is nearly impossible. I know the Americans don't have them."
That's a key tool that authorities currently lack. Salah Abdeslam, a suspect in the Paris attacks suspect, seemingly hid in plain sight just blocks away from the Brussels neighborhood where he grew up.
Authorities captured him after a gun battle last week, but would they have been able to arrest him sooner if they had the type of sources Baer thinks they lack?
Maybe not, CNN's Robertson says.
"As once Holland fought tides and rain, today Belgium has been trying to hold back another force of nature: A disaffected, disassociated youth, warped and wrapped in ISIS's corrosive ideology," he says. "Their numbers have been too great for Belgian counterterrorism efforts to cope."
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.
"These two deceased suicide bombers had lengthy criminal records," Van Leeuw said Wednesday, "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 that 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.
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.
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. It indicated Bakraoui "needs to rush" and "no longer feels safe."
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 was one of the Brussels airport bombers.
Abdeslam 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.
And a Belgian security source said Khalid El Bakraoui rented the apartment where Abdeslam was hiding.
Clues from 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.