An image from a security camera shows three suspects of the attacks at Brussels Airport, in Zaventem, pushing trollies with suitcases. The Belgian federal prosecutor confirmed, April 9, that Mohamed Abrini, right, who was arrested in connection to the attacks in Paris on Nov. 13, 2015, was the “man in the hat” captured in surveillance footage at the airport on March 22.

March 22, 2016 An image from a security camera shows three suspects of the attacks at Brussels Airport, in Zaventem, pushing trollies with suitcases. The Belgian federal prosecutor confirmed, April 9, that Mohamed Abrini, right, who was arrested in connection to the attacks in Paris on Nov. 13, 2015, was the “man in the hat” captured in surveillance footage at the airport on March 22. Belgian Federal Police/AP

More than a dozen people were killed, and several others were injured, after explosions at an airport and metro station in the Belgian capital.

At least 31 people were killed and 270 injured in suicide bombings at an airport and metro station in the Belgian capital.

At least 31 people were killed and 270 injured in suicide bombings at an airport and metro station in the Belgian capital.

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, according to 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.

[A quiet morning in Brussels ends in gruesome terrorist attacks]

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 missive, 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.

Belgium authorities have released more details about the suspects in the deadly terrorist attacks in Brussels. (Claritza Jimenez/The Washington Post)

[Why is Brussels under attack?]

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, according 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.

European security leaders planned to gather Thursday in Brussels to discuss whether to pursue new policies that would better pool information to counter terrorism.

French Prime Minister Manuel Valls, visiting Brussels on Wednesday to extend his condolences, repeated past calls for sweeping new powers to be given to European intelligence agencies. “In the years to come, the [E.U.] member states will have to invest massively in their security systems,” he said.

[Brussels terrorists probably used explosive nicknamed ‘the Mother of Satan’]

Van Leeuw, the Belgian prosecutor, said the brothers had not previously been suspected of ties to terrorism.

Turkish President Recep Tayyip Erdogan said Wednesday that Turkey had deported one of the attackers to Europe in July and warned European counter­terrorism officials that it believed the man was a militant, suggesting a serious lapse by Belgian authorities. Interpol had also issued a “red notice,” effectively an international arrest warrant, for one of the suspects at the request of Belgian authorities. It was not immediately clear when that notice had been issued.

There were signs that an even bigger attack had been forestalled. Authorities found large stockpiles of bomb-building materials at Ibrahim el-Bakraoui’s apartment in the Schaerbeek area of Brussels, the prosecutor said: 33 pounds of TATP explosives, nearly 40 gallons of acetone, 8 gallons of hydrogen peroxide, detonators, and a suitcase full of nails and screws. Both acetone and hydrogen peroxide are easily obtainable; together they can be used to make potent explosives.

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 — without providing more specific figures. He said that U.S. diplomatic missions in Brussels were working to account for all of their own staff.

Secretary of State John F. Kerry plans to visit Brussels on Friday on his return from a trip to Moscow.

Griff Witte, Missy Ryan, James McAuley and Anthony Faiola in Brussels and Brian Murphy and William Branigin in Washington contributed to this report.

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