BRUSSELS (AP) -- Islamic extremists struck Tuesday in the heart of Europe, killing at least 34 people and wounding scores of others in back-to-back bombings of the Brussels airport and subway that again laid bare the continent's vulnerability to suicide squads.

Bloodied and dazed travelers staggered from the airport after two explosions - at least one blamed on a suicide attacker and another apparently on a suitcase bomb - tore through crowds checking in for morning flights. About 40 minutes later, another rush-hour blast ripped through a subway car in central Brussels as it left the Maelbeek station, in the heart of the European Union's capital city.

Authorities released a photo taken from closed-circuit TV footage of three men pushing luggage carts in the airport, saying two of them apparently were suicide bombers and that the third - dressed in a light-colored coat, black hat and glasses - was at large. They urged the public to reach out to police if they recognized him. The two men believed to be the suicide attackers apparently were wearing dark gloves on their left hands, possibly to hide detonators.

In police raids Tuesday across Belgium, authorities later found a nail-filled bomb, chemical products and an Islamic State flag in a house in the Schaerbeek neighborhood, the state prosecutors' office said in a statement.

In its claim of responsibility, the Islamic State group said its members detonated suicide vests both at the airport and in the subway, where many passengers fled to safety down dark tunnels filled with hazy smoke from the explosion. A small child wailed, and commuters used cell phones to light their way out. IS warned of further attacks, issuing a communique promising "dark days" for countries taking part in the anti-IS coalition, warning

The government said at least 11 people were killed at the airport and 20 on the subway, where the bomb hit an enclosed train car. Later, a security official said the overall death toll had risen to 34, without providing a breakdown of where.

European security officials have been bracing for a major attack for weeks and warned that IS was actively preparing to strike. The arrest Friday of Salah Abdeslam, a key suspect in the Nov. 13 attacks in Paris, heightened those fears, as investigators said many more people were involved than originally thought and that some are still on the loose.

Shockwaves from the attacks crossed Europe and the Atlantic, prompting heightened security at airports and other sites.

Belgium raised its terror alert to the highest level, shut the airport through Wednesday and ordered a city-wide lockdown, deploying about 500 soldiers onto Brussels' largely empty streets to bolster police checkpoints. France and Belgium both reinforced border security.

Medical officials treating the wounded said some victims lost limbs, while others suffered burns or deep gashes from shattered glass or suspected nails packed in with the explosives. Among the most seriously wounded were several children.

The bombings came barely four months after suicide attackers based in Brussels' heavily Muslim Molenbeek district slaughtered 130 people at a Paris nightspots, and intelligence agencies had warned for months a follow-up strike was inevitable. Paris fugitive Abdeslam was arrested in Molenbeek.

Abdeslam has told investigators he was planning to "restart something" from Brussels, said Belgian Foreign Minister Didier Reynders. He said Sunday that authorities took the claim seriously because "we found a lot of weapons, heavy weapons in the first investigations and we have seen a new network of people around him in Brussels."

Officials at the airport in the Brussels suburb of Zaventem said police had discovered a Kalashnikov assault rifle and an explosives-packed vest abandoned at the facility, offering one potential lead for forensic evidence. Bomb disposal experts safely dismantled that explosive device.

The U.S. State Department warned Americans about potential risks to travel in Europe, issuing an advisory saying that "terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants and transportation." It advised U.S. citizens to "exercise vigilance" in public places or on mass transportation.

Several Americans were among the wounded, including an Air Force lieutenant colonel stationed in the Netherlands, his wife and four children who were at the airport. Mormon church officials, meanwhile, said three of its missionaries from Utah were seriously injured in the airport blasts and were hospitalized.

Three intelligence officials in Iraq told the AP that they had warned European colleagues last month of IS plans to attack airports and trains, although Belgium wasn't specified as a likely target. The officials, who monitor activities in the IS stronghold of Raqqa, Syria, said Brussels may have become a target because of the arrest of Abdeslam.

One of the officials - all of whom spoke on condition of anonymity because they were not authorized to talk about their knowledge of IS operations - said Iraqi intelligence officials believe that three other IS activists remain at large in Brussels and are plotting other suicide-bomb attacks.