The four men — two of them brothers — who turned ordinary morning commutes in Brussels into blood-soaked nightmares may have been spurred into action by fears that authorities were closing in on them, according to a note left by one of the attackers that was described by a prosecutor Wednesday.
Days before the attacks on Tuesday, counterterrorism police had raided their Brussels safe houses. An ally who took part in November's Paris carnage was shot and captured by authorities. And Ibrahim el-Bakraoui, a 29-year-old Belgian with a thick rap sheet, wrote that he did not want to wind up in a prison cell, Belgian federal prosecutor Frederic Van Leeuw said Wednesday.
The men — at least two of whom had direct ties to the Islamic State attacks in Paris — knew they had to act decisively. So they set out with explosives that ripped open a Brussels subway car and shattered the city's main airport terminal, killing at least 31 people and injuring 300 in the bloodiest attack on Belgian soil since World War II.
Bakraoui detonated a suitcase full of nails, screws and powerful explosives at the airport, killing himself in the process, Van Leeuw said. So did Islamic State bombmaker Najim Laachraoui, 24, who is also believed to have prepared explosives for the Paris attacks, the Washington Post reported, citing an Arab intelligence official and a European intelligence official who spoke on the condition of anonymity because they were not authorized to discuss the matter publicly.
An unidentified man who left an even larger suitcase of explosives at the airport is believed to still be at large, he said. That suitcase did not immediately detonate, sparing Belgium even more casualties.
The country held a national minute of silence Wednesday led by Prime Minister Charles Michel, who laid a wreath at the Maelbeek metro station in honor of the victims. Thousands of Belgians gathered in a somber ceremony in front of an ornate 19th-century stock exchange building to light candles and lay flowers.
The bomber's note, contained in a computer that had been chucked into a garbage can near Bakraoui's Brussels apartment, does not specifically cite recent raids across Belgium, including one that netted a key suspect in the Paris attacks. But its tone suggests a sense that the noose was tightening, Van Leeuw said.
The computer message also gives apparent insight into the organization and motivation of militants who apparently turned their attention to Brussels after pulling off the Paris attacks that killed 130 people.
In the note, Bakraoui described feeling pressure bearing down. He wrote that he was "in a hurry, no longer knowing what to do, being searched for everywhere, no longer secure," according to Van Leeuw's description of the message, which was not made public.
Laachraoui's involvement draws the boldest line yet between the Paris attacks and those in Brussels. His DNA was found on explosives in the Paris attacks, and authorities believe that he was versed in the Islamic State art of assembling powerful explosives from ingredients that are readily available. His participation in two attacks suggests that the Islamic State is increasingly able to strike on European soil — although his death may also mean that he feared imminent capture by European authorities.
Terrorism experts regard bombmakers, especially those trained in handling sensitive explosives, as among the most valuable and protected members of a terrorist organization. It is highly unusual for them to participate in suicide attacks themselves.
Laachraoui's DNA was found in a Brussels apartment raided last week. The discovery of a militant cell there eventually led to the arrest of Salah Abdeslam on Friday. Abdeslam was the final at-large direct participant in the Paris attacks and is believed to have been the logistics mastermind.
The computer file that prosecutors cited Wednesday does not mention Abdeslam by name, but it says the attackers feared that if they did not strike quickly, they risked winding up in prison alongside "him."
"If they drag on, they risk finishing next to him in a cell," Van Leeuw said, paraphrasing the contents of the file.
Van Leeuw described the file as a "will" discovered on a computer. He did not explain why authorities believed the computer belonged to Bakraoui.
Bakraoui's younger brother, Khalid el-Bakraoui, 27, is believed to have been the suicide bomber on a Brussels subway car that blew up as it sped out of a station underneath the heart of the European Union quarter of Brussels, an area packed with embassies and international organizations. That attack came 73 minutes after the one at the airport, meaning that commuters were already reading the news of the first explosions when the carnage reached them.
Khalid el-Bakraoui appears to have been a kind of surreptitious real estate broker for the plotters, the Post reported, citing to a European security official who spoke on the condition of anonymity because he was not authorized to discuss the case. Using assumed names, he rented an apartment in the Forest area of Brussels where Abdeslam's fingerprints were found and an apartment near Charleroi, Belgium, where Paris attack mastermind Abdelhamid Abaaoud stayed as he plotted the violence.
Both Bakraoui brothers served prison time for violent crime, the European security official said. The announcement on Wednesday that two of the attackers were brothers highlighted another emerging tactic from the militant group: They would be the third pair of brothers involved in an Islamic State attack in Europe in the past 15 months.
It remained unclear Wednesday how many Americans had been killed in the blasts. In Washington, State Department deputy spokesman Mark Toner said that "approximately a dozen" Americans were injured but that "a number" of U.S. citizens remained unaccounted for on Wednesday.
Secretary of State John F. Kerry plans to visit Brussels on Friday on his return from a trip to Moscow.