By Jan Strupczewski, Julia Fioretti and Alastair Macdonald
BRUSSELS (Reuters) - Belgium's chief prosecutor named two brothers on Wednesday as Islamic State suicide bombers who killed at least 31 people in the most deadly attacks in Brussels' history but said another key suspect was on the run.
Tuesday's attacks on a city that is home to the European Union and NATO sent shockwaves across Europe and around the world, with authorities racing to review security at airports and on public transport. It also rekindled debate about lagging European security cooperation and flaws in police surveillance.
The federal prosecutor told a news conference that Ibrahim El Bakraoui, 29, one of two men who blew themselves up at Brussels airport on Tuesday, had left a will on a computer dumped in a rubbish bin near the militants' hideout.
In it, he described himself as "always on the run, not knowing what to do anymore, being hunted everywhere, not being safe any longer and that if he hangs around, he risks ending up next to the person in a cell" - a reference to suspected Paris bomber Salah Abdeslam, who was arrested last week.
His brother Khalid El Bakraoui, 27, detonated a bomb an hour later on a crowded rush-hour metro train near the European Commission headquarters, prosecutor Frederic Van Leeuw said.
Both men, born in Belgium, had criminal records for armed robbery but were not previously linked by investigators to Islamist militants.
At least 31 people were killed and about 271 wounded in the attacks, the prosecutor said. That toll could increase further because some of the bomb victims at Maelbeek metro station were blown to pieces and victims are hard to identify. Several survivors were still in critical condition.
The Bakraoui brothers were identified by their fingerprints and on security cameras, the prosecutor said. The second suicide bomber at the airport had yet to be identified and a third man, whom he did not name, had left the biggest bomb and run out of the terminal before the explosions.
Belgian media named that man as Najim Laachraoui, 25, a suspected Islamic State recruiter and bomb-maker whose DNA was found on two explosives belts used in last November's Paris attacks and at a Brussels safe house used by Abdeslam before his arrest last Friday.
Some media reported he had been captured in the Brussels borough of Anderlecht, but they later said the person detained was not Laachraoui.
Khalid El Bakraoui had rented under a false name the apartment in the city's Forest borough, where police hunting Abdeslam killed a gunman in a raid last week. He is also believed to have rented a safe house in the southern Belgian city of Charleroi used to mount the Paris attacks.
"BLACK DAYS"
The Syrian-based Islamist group claimed responsibility for Tuesday's attacks, warning of "black days" for those fighting it in Syria and Iraq. Belgian warplanes have joined the coalition in the Middle East, but Brussels has long been a center of Islamist militancy.
A minute's silence was observed across Belgium at noon. Prime Minister Charles Michel canceled a trip to China and reviewed security measures with his inner cabinet before attending a memorial event at European Commission headquarters with King Philippe, Commission President Jean-Claude Juncker and French Prime Minister Manuel Valls.
"We are determined, admittedly with a strong feeling of pain in our stomachs, but determined to act," Michel told a joint news conference with Valls. "France and Belgium are united in pain more than ever."
Valls played down cross-border sniping over security, saying: "We must turn the page on naivete, a form of carefreeness that our societies have known.
"It is Europe that has been attacked. The response to terrorism must be European."
More than 1,000 people gathered around an improvised shrine with candles and street paintings outside the Brussels bourse.
Belgium's crisis coordination center kept the level of security alert at the maximum as the man hunt continued. Some buses and trains were running but the metro and the airport were closed, along with key road tunnels in Brussels.
The blasts fueled political debate across the globe about how to combat militants.
"We can and we will defeat those who threaten the safety and security of people all around the world," said U.S. President Barack Obama.
Donald Trump, the front-runner for the Republican nomination to succeed Obama in November's U.S. election, suggested suspects could be tortured to avert such attacks.
After a tip-off from a taxi driver who unknowingly drove the bombers to the airport, police searched an apartment in the Brussels borough of Schaerbeek late into the night, finding another bomb, an Islamic State flag and bomb-making chemicals.
An unused explosive device was later found at the airport.
CLOSING IN
Security experts believed the blasts were probably in preparation before Friday's arrest of locally based French national Abdeslam, 26, whom prosecutors accuse of a key role in the Nov. 13 Paris attacks.
He was caught and has been speaking to investigators after a shootout at an apartment in the south of the city, after which another Islamic State flag and explosives were found.
It was unclear whether he had knowledge of plans for Tuesday's attack or whether accomplices precipitated their action, fearing police were closing in.
About 300 Belgians are estimated to have fought with Islamists in Syria, making the country of 11 million the leading European exporter of foreign fighters and a focus of concern in France and other neighbors over its security capabilities.
Reviving arguments over Belgian policies following the Paris attacks, in which 130 people were killed in an operation apparently organized from Brussels, French Finance Minister Michel Sapin spoke of "naiveté" on the part of "certain leaders" in holding back from security crackdowns on Muslim communities.
Belgian Foreign Minister Didier Reynders retorted that each country should look to its own social problems, saying France too had rough high-rise suburbs in which militants had become radicalized. Valls said France had no place teaching Belgium lessons and had its own security problems.
Brussels airport seemed likely to remain shut for several days over the busy Easter holiday weekend, since the departure hall was still being combed as a crime scene on Wednesday and repairs can only begin once investigators are finished.