Four men slipped away after the November attacks in Paris, and all but one were part of the Isis (Islamic State) cell that attacked Brussels.
Some of the figures in the Paris plot had become "cannon fodder" by the time of the Brussels attacks, said Nicolas Henin, a journalist held hostage by Isis for 10 months. "Once they had performed their services in Paris, they were considered expendable," he wrote. "That is how Isis works in terms of human resources."
The bomb maker
With a degree in mechanical engineering, Najim Laachraoui left for Syria in February 2013. He returned home as an expert in urban explosives, bearing a fake Belgian ID and an alias: Soufiane Kayal. On September 9, 2015, police at the Austro-Hungarian border briefly stopped a Mercedes and sent him on his way. In the car with him were Salah Abdeslam and another Isis recruit with a fake Belgian ID.
Laachraoui's job in the Paris attacks was to manufacture the TATP explosives and the suicide vests. His DNA was found on two vests. Thirteen days later, that same DNA was found in a safe house in the Belgian town of Auvelais, and authorities soon linked the name Kayal to another apartment in Charleroi, where some of the November attackers stayed.
The cell, meanwhile, appears to have found another place for making TATP: a top-floor flat in the Brussels suburb of Schaerbeek.
In the apartment, there was enough TATP for three suitcase bombs - 15kg to 20kg each. There were also 15kg left over and enough base materials to make 100kg more.
Why Laachraoui went from bomb maker to suicide bomber at Brussels' airport remains unclear. Had the group found a replacement? Had he become too much of a liability as police closed in on his trail? "It's strange," said Patrick Skinner, a former CIA case officer with the Soufan Group. "They don't have a shortage of people that are willing to become a walking bomb, but there's always a shortage of talent. It's like having General Eisenhower lead the charge at D-Day. It's possible but it seems to be a supreme waste of talent."
The transporter
Salah Abdeslam drove thousands of kilometres across Europe over months, collecting accomplices, scouting locations and buying equipment. He rented apartments and cars and, on November 12, drove towards Paris with another one-time petty criminal and boyhood friend, Mohamed Abrini.
Abdeslam ditched the car in northern Paris and is believed to have discarded his unexploded suicide vest south of the city. And he called on two friends from Brussels' Molenbeek neighbourhood to drive through the night to pick him up.
"I think he had no desire to die, and he had other things to do," said Nathalie Goulet, a French senator and co-president of a commission that studied jihadi networks. "They escape, they gather back home, they resume, because that's their job."
Abdeslam was flushed out on March 16, when police went to search what they thought was a vacant apartment in the Forest neighbourhood of Brussels and instead were sprayed with gunfire.
By March 19, police had traced him to another hideout - this time just around the corner from Molenbeek. He was shot in the leg as he tried to escape yet again.
The landlord
Khalid El Bakraoui, who died in the metro attack, had an alias and a warrant out for his arrest. He and his brother Ibrahim, one of the airport bombers, were known criminals, bank robbers and car thieves.
Interpol issued an international warrant for his arrest in December - soon after it was discovered he had rented the Charleroi apartment that served as a departure point for some of the Paris killers. El Bakraoui scouted locations for the network's growing number of accomplices. It was El Bakraoui who rented the Forest safe house and was seen leaving the Schaerbeek apartment.
The cipher
Mohamed Abrini, a 31-year-old Belgian petty criminal and boyhood friend of Abdeslam, is believed to have travelled last year to Syria, a short trip to the country where his younger brother died in 2014.
He went multiple times to Birmingham, England, meeting with several men suspected of terrorist activity.
Abrini's role in the subsequent weeks and months while the Paris attackers were coming together has never been clarified. He has been on the run. Like Abdeslam, he had ties to Abdelhamid Abbaoud, the charismatic ringleader of the Paris attacks who died in a police standoff . He is the last identified suspect still at large from the November attacks.
