(CNN) -- ISIS claimed to strike yet again on European soil Tuesday, saying its "fighters" launched attacks on the airport and a subway station in Belgium's capital that killed at least 34 people and injured more than 200.
The atmosphere in Belgium has been tense for months, with the authorities warning of possible threats and pursuing terrorists. Tuesday's attacks followed on the heels of last week's capture of Europe's most wanted man, Salah Abdeslam, in a bloody raid in Brussels.
"We were fearing terrorist attacks," Belgian Prime Minister Charles Michel told reporters Tuesday. "And that has now happened."
A Belgian government representative told CNN that 20 people died at the Maelbeek metro station and 130 were wounded, plus 10 more were killed and 100 wounded at Brussels' international airport.
The "working assumption" is that the attackers came from the network behind November's massacres in Paris, which left 130 dead, Belgian security sources said, while cautioning it is very early in the latest investigation.
After Tuesday's attacks, Belgian state broadcaster RTBF reported that Belgian authorities carried out midday raids in a search for people linked to the attacks. Several witnesses told CNN they'd seen police special forces combing through the northeast Brussels neighborhood of Schaerbeek, cordoning off a train station there.
Investigators found a nail bomb, chemical products and an ISIS flag during a house search in the neighborhood, Belgium's federal prosecutor said in a statement.
Belgian police released a notice that includes a photograph of a suspect "wanted (for) terrorism" and asks the public, "Who recognizes this man?"
Federal Prosecutor Frederic Van Leeuw said the man is one of three shown in surveillance images pushing airport luggage carts alongside each other. Two of them "probably carried out suicide attacks," he said, while the third -- the one in light clothing, glasses and a hat -- "is actively being searched for."
As it has for other terrorist attacks in Europe, Asia and Africa, ISIS embraced all the assailants. Its claim noted that Belgium is "participating in the international coalition against the Islamic State." Belgian warplanes flew 796 sorties and launched 163 airstrikes over Iraq from September 2014 to July 2015, according to the U.S.-led anti-ISIS coalition, and were set to resume these operations this summer.
Johan Verbeke, Belgium's U.S. ambassador, insisted Tuesday that his country will remain an active participant in the coalition, saying, "There is no way that intimidation will be rewarded."
But just as Belgium plans to continue its anti-ISIS operations, the extremist group may not be done attacking that country and others. One Twitter post widely circulated by prominent ISIS backers featured the words, "What will be coming is worse."
A senior U.S. official thinks it's no coincidence that ISIS would want to strike in Brussels, home to the headquarters of NATO and the European Union.
"They are trying to make an international statement," the official said. "They aren't stupid."
'It was a matter of time'
Belgium is no stranger to terror. A U.S. counterterrorism official said, "The Belgians have been sitting on a ticking time bomb," given all those who have gone from the small European nation to Syria and Iraq to join ISIS, then possibly come back home.
Still, these facts don't take away the shock and horror of those who lived through Tuesday's carnage.
"You cannot believe it; you cannot believe it," said Jef Versele, who was in the airport's departure hall when bombs exploded there. "It was so insane. Not in my backyard."
One of the two airport explosions happened outside security checkpoints for ticketed passengers and near the airline check-in counters, an airline official briefed on the situation said.
The subway station blast happened in the Brussels district of Maelbeek, near the European Quarter, where European Union institutions are based.
Richard Medic, who arrived at the metro station shortly after that explosion, wasn't surprised by the carnage after all that Europe has gone through recently, including November's massacre in Paris that ISIS claimed responsibility for.
"I think, after the Paris attacks, we were assuming something like this would happen," the Brussels resident told CNN. "And it was a matter of time."
Yet Versele, the airport witness, said he thinks Belgians should not hole themselves up.
Instead, he said, they should continue to live their lives and travel "to prove that we're not afraid of those who have done (the attacks)."
Two nuclear power plants evacuated
Belgian authorities bolstered security after Tuesday's attacks, including shutting down all Brussels metro stations and evacuating the city's airport.
This comes as the terror threat level in Belgium went up to four -- its highest. That step-up means army soldiers can be sent onto the streets to meet security needs.
No flights will go into or out of Brussels Airport -- which CEO Arnaud Feist said was "shot in the heart" -- until at least noon Wednesday, according to Belgium's crisis center.
Several of the city's top shopping centers are closed for Tuesday and perhaps beyond, according to broadcaster RTBF, while train stations reopened at 4 p.m. "with extra security measures."
All but essential staff were sent home from two nuclear power plants in Belgium -- one in Tihange and the other in Doel, said a representative of Engie, the French company that operates the facilities. Belgian authorities ordered this evacuation, though the representative did not provide further details.
And RTBF reported that the National Pensions Office in Brussels was cleared after two suspicious packages were found inside.
Amid the special precautions, investigations and sadness, including three days of national mourning, Belgium's Prime Minister offered a resolute message to those who supported and cheered the attackers.
"To those who have chosen to be barbarous enemies of freedom, democracy and fundamental values ... we remain united as one," Michel said. "We are determined to defend our freedoms and to protect our liberties."
European calls for solidarity
NATO, the military alliance that calls Brussels home, increased its own alert level and expressed solidarity with Belgium.
"This is a cowardly attack, an attack on our values and on our open societies," NATO Secretary General Jens Stoltenberg said in a statement. "Terrorism will not defeat democracy and take away our freedoms."
As in other places -- as far away as New York, Washington, Chicago and Los Angeles in the United States -- British authorities ordered an increased police presence at ports, airports, Tube stations and international train stations, according to Prime Minister David Cameron.
Eurostar, a high-speed railway that goes to England and France, noted a number of schedule and other changes, including canceling service between London and Brussels.
Throughout France, 1,600 more police hit the streets after the Brussels attacks, French Interior Minister Bernard Cazeneuve said.
"I express my entire solidarity with the Belgian people," French President Francois Hollande tweeted. "Through the Brussels attacks, the whole of Europe has been hit."
That sentiment was echoed by Belgium's Michel during what he called "a dark time for our country."
"More than ever, I call on everybody to show calm, but also solidarity," the Prime Minister said. "We are facing a difficult, challenging time. And we should face up to this challenge by being united."
Airport witness: Windows broken, ceiling parts down
Around 8 a.m. Tuesday, Alphonse Lyoura was doing his job handling bags at Brussels Airport when, he said, he heard someone speaking and then "a huge, strong explosion."
"It's horrible," he told CNN affiliate BFM, recalling a man who'd lost both legs and other grisly scenes. "Belgium doesn't deserve this."
In the departure hall, Versele noticed "people ... shouting and running around" after the first blast, then even more pandemonium after the second explosion, which was, "in my eyes, much more powerful."
That blast blew out windows, created a lot of smoke and caused parts of the ceiling to fall, he added.
"People were on the floor," Versele said, estimating that he saw 50 to 60 who didn't seem to be able to walk. "... It was quite a mess."
Jeffrey Edison was near the gate, several hundred yards from where the explosions occurred. He didn't hear the blasts but "suddenly saw" 200 to 300 people rushing toward him from a security checkpoint.
Anthony Barrett watched it unfold from his hotel across from the terminal building.
"When I opened the curtains and looked out, I could see people fleeing," he told CNN -- some of the wounded being carried out on stretcher after stretcher, others on luggage carts.
Soccer star: 'I wish for Brussels to act with dignity'
About an hour later, during the tail end of Brussels' rush hour, another blast went off at the Maelbeek metro station.
Sander Verniers was heading toward there when this explosion occurred, producing "strong winds going (through his train) and some noises that shouldn't be there." Belgian troops met the passengers as they got off the train and walked along the tracks.
"We all had to get out," Verniers told CNN. "There was a lot of smoke."
For several hours Tuesday, everyone in Belgium was urged to stay inside. That advisory was lifted around 4 p.m., though the country's crisis center urged all citizens to "remain vigilant."
One question is for how long. Another is what this has to do with Belgium's other suspected terrorists like Abdeslam, who reportedly told investigators he was involved in last fall's Paris terror attacks that ISIS also claimed responsibility for.
The Belgium-born French citizen was arrested in the Brussels suburb of Molenbeek on Friday. On Tuesday, Belgium's Prime Minister deflected a question about whether there's any link between the Brussels attacks and Abdeslam's capture, saying it is too early to tell.
Michel said Tuesday that he had "no information" about who was responsible for the attack, adding that authorities will find that out, but right now their focus is on caring for the victims.
Belgian national soccer team captain Vincent Kompany tweeted that he was "horrified and revolted (that) innocent people (are) paying the price again," but he urged people not to encourage those wishing to lash out.
"We must reject hate and its preachers," Kompany said. "... I wish for Brussels to act with dignity."