Kim Hjelmgaard, Delphine Reuter and John Bacon

USA TODAY

BRUSSELS — The Islamic State claimed responsibility for brutal attacks that brought blood and chaos to this capital city's airport and downtown metro station Tuesday, killing dozens of people, wounding more than 150 and heightening terror alerts around the world.

As of Tuesday evening, the death toll was 34, the Associated Press said a Belgian security official indicated.

Authorities blamed suicide bombers for the attacks, but embarked on manhunt for at least one suspected surviving attacker.

However, a federal law enforcement official said Tuesday there was no immediate reason to doubt the Islamic State’s claim of responsibility.

The official who is not authorized to comment publicly said that based on the coordinated nature of the attacks it was likely that the assaults had been long planned but were accelerated following last week’s arrest of the Paris suspect Salah Abdeslam. The official discounted the notion that the attacks were a form of retaliation. Rather, the official said, it appeared that they may have been accelerated out of concern that the plots might be compromised if Abdeslam was cooperating with Belgian investigators.

It was earlier Tuesday that two deadly explosions tore through the departures hall at Brussels Airport in nearby Zaventem, Belgium's federal prosecutor Frederic Van Leeuw said. A short time later, a third blast brought mayhem and death to the Maelbeek metro station, near European Union institutions in central Brussels, he said.

"We were fearing terrorist attacks and that has now happened," Belgian Prime Minister Charles Michel said.

Federal health officials said the airport attack killed at least 11 people and injured 81. Brussels Mayor Yvan Majeur put the subway death toll at 20, with more than 100 wounded there.

The Islamic State claimed responsibility for the attacks in a statement to the Amaq news agency.

"Islamic State fighters opened fire inside Zaventem airport before several of them detonated their explosive belts," the statement said. A "martyrdom bomber detonated his explosive belt in the Maelbeek metro station," it added.

The attack came four days after the arrest of Salah Abdeslam, a suspect in November's Paris attacks who was apprehended in Brussels after a four-month manhunt. After the arrest, authorities said Abdeslam had planned to commit another attack, and had a large network of associates. Still, authorities said it was too soon in the investigation to tie Tuesday's horror to the Paris terror strike.

Belgium raised the terror threat to its maximum level as security teams swept neighborhoods for collaborators. The prosecutor's office released a photo taken from an airport security camera showing possible suspects before the blasts.

Police found an explosive device containing nails, "chemical products" and an Islamic State flag during a house search in the Brussels neighborhood of Shaerbeek, the Associated Press reported.

Authorities shut down public transportation, closed the airport and warned residents to stay home. Non-essential staff were evacuated from Belgium's power plants in Doel and Tihange as a precaution, the plants' French operator, Engie, told Reuters news service.

By late Tuesday, some trains stations reopened to long lines as people began emerging from their homes. Authorities said the airport would remain closed through Wednesday.

President Obama, speaking in Havana on the third day of a Latin American tour, said the U.S. stands with Belgium and that the attack is "yet another reminder that the world must unite, we must be together regardless of nationality or race or faith, in fighting against the scourge of terrorism." Later, he ordered all American flags in the U.S. to be flown at half-staff through Saturday.

Authorities in Los Angeles, New York, Washington, Boston and elsewhere across the nation tightened security at transit hubs and tourist sites. There were no indications of specific threats against U.S. targets, authorities said.

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In Belgium, RTBF, citing the federal prosecutor's office, said Arabic screams were heard in the departures hall before shots rang out, followed by the explosions. Weapons were found at the scene, the broadcaster reported.

Three Mormon missionaries from Utah were seriously injured in the airport blasts, the Church of Jesus Christ of Latter-day Saints said. The Belgian Crisis Center said a suspicious package was "neutralized" by security officials shortly after the attack.

The U.S. Air Force said one of its servicemembers from Joint Force Command Brunssum, the Netherlands, was injured in the airport attack. The airman's family was also present and sustained various injuries, the military branch said in a statement.

Airport worker Anthony Deloos told AP the first explosion took place near counters where customers pay for overweight baggage. He and a colleague said the second blast hit near a Starbucks.

"We heard a big explosion. It’s like when you’re in a party and suddenly your hearing goes out, from like a big noise,” Deloos said, adding that he jumped into a luggage chute to protect himself.

The airport evacuation left hundreds of people stuck in the parking lot and on the runway of the airport.

Simon O’Connor, who works for the European Commission in Brussels, told the Financial Times he was in the lot when he heard the first explosion and thought it had something to do with construction work. He heard the second explosion minutes later.

“I walked to the edge of the car park and you could see a lot of people looking distraught,” he said. “The whole side of the terminal building was blown out. A lot of people had blood on their faces, leg injuries. People began pouring out of the building.”

Immediately following the blast at the metro station, transportation authorities shut down the entire subway system. Thirty minutes later, officials suspended all public transportation, including trams and buses.

Ángela González, 45, lives near an underground station in Brussels.

"Schools are closed, no one is allowed to leave. In fact, no one is allowed out of wherever they are," she said. "All you can hear is sirens and streets are completely deserted."

González said the attacks don't make her want to return to her native Spain. "This is not just a Belgium problem, it is a European problem. We would not be safer elsewhere."

The EU, headquartered in Brussels, told its staff to stay at home or remain inside buildings. The location of the metro stop is in a main thoroughfare of Brussels that connects the Schuman roundabout, where the European Commission and the Council of the EU are located, to the center of the city.

Michel also urged people to stay at home. He called the attacks a "black day for Belgium" and urged calm. "We have to face this united," he said.

The attack drew a call for unity from numerous nations. Russian President Vladimir Putin said the attack showed the need for a global approach to combating terror. Italian premier Matteo Renzi pressed for a common European strategy for security and defense.

“Europe has to go all the way this time," he said.

In Paris, where terror attacks on Nov. 13 killed 130, French President Francois Hollande said the war against terror must be fought "in cold blood."

"Through the attacks in Brussels, the whole of Europe has been hit," Hollande said.

"We are at war," said French Prime Minister Manuel Valls. "We have been enduring acts of war for many months in Europe. And in the face of this war, we need an every minute mobilization."

Hjelmgaard reported from Berlin; Bacon from McLean, Va. Contributing: Belen Diego and Maya Vidon from Berlin.

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