GWEN IFILL: Once again, a major European capital has been shut down by terror. Today's bombings in Brussels left at least 31 people dead. More than 180 were wounded, including an undetermined number of Americans.

The attacks paralyzed the city for most of the day, and triggered an all-out manhunt.

Thick smoke and screams of panic captured by a cell phone moments after double explosions rocked the Belgian capital's airport. Suitcases stood abandoned as would-be passengers scrambled over collapsed ceilings and shattered windows.

WOMAN: I hear an explosion. And all the ceilings is going down. And then I just go under the sink. And then the second explosion went. And then everything is black. And I see — when I go out, I see all — a lot of people with blood.

MAN (through interpreter): I tried to flee towards the arrivals and we couldn't get there, and there was glass everywhere, crash, crash, crash everywhere. And we went toward the taxis. It was a total confusion. It was truly hell.

GWEN IFILL: Belgian prosecutors say it was the work of two suicide bombers, attacking just seconds apart, during the morning rush. They say a third suspect escaped. Passengers and employees alike streamed out of the smoldering entrance, while hundreds of others were evacuated to the tarmac.

Then, about an hour later, another blast, this time on a subway train as it left a station near the European Union's headquarters. Footage posted to social media showed passengers evacuating one car as emergency personnel treated the wounded.

MAN (through interpreter): These are war injuries. I have more than 40 years of experience on the job, so I have seen a lot of things. I think this is the worst thing I have ever seen in my career.

GWEN IFILL: The twin attacks, claimed by the Islamic State group, sent Belgium's terror alert to the highest level and put the entire city under lockdown for hours. Police found a third bomb at the airport and disposed of it.

Flights and train service were canceled or diverted, and people were ordered to remain in place.

CHARLES MICHEL, Prime Minister, Belgium (through interpreter): Ladies and gentlemen, what we feared has happened. Our country and our citizens have been hit by blind, violent and cowardly attacks. We are confronted with a challenge, a difficult challenge, and we must face it by standing united, showing solidarity and staying together.

GWEN IFILL: The attacks in Brussels came four days after the arrest there of Salah Abdeslam, the top suspect in November's terror attacks in Paris that left 130 dead.

And they cast new doubts on Belgian intelligence-gathering and security.

The "NewsHour"'s chief foreign affairs correspondent, Margaret Warner, spoke with the Belgian ambassador to the United States in Washington today

JOHAN VERBEKE, Ambassador, Belgium: Some indications pointed to the fact that something may happen. The security levels were heightened. The presence of security people, including military, quite visible, had been enhanced. And still it happened, so the answer to your question is, it can happen even when you take all security preparedness.

GWEN IFILL: World leaders, including President Obama, pledged solidarity with the Belgians. He learned of the attacks on the second day of his visit to Cuba.

PRESIDENT BARACK OBAMA: We will do whatever is necessary to support our friend and ally Belgium in bringing to justice those who are responsible, and this 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.

GWEN IFILL: And in Paris, President Francois Hollande urged leaders everywhere to recognize the danger.

PRESIDENT FRANCOIS HOLLANDE, France (through interpreter): Terrorism hit Belgium, but Europe was targeted, and the whole world is concerned. We are facing a global threat, which demands a global response.

GWEN IFILL: The immediate response in Britain, France, Germany and elsewhere, was to bolster security at airports, train stations and border crossings.

In the U.S., similar measures were taken in major urban areas, including Washington and New York.

MAYOR BILL DE BLASIO (D), New York City: Let me say at the outset, there is no specific and credible threat against New York City at this time, but we are in a high state of vigilance and readiness.

GWEN IFILL: The Brussels Airport, meanwhile, will stay closed at least through Wednesday.

Later today, President Obama ordered U.S. flags lowered to half-staff at government buildings. And the Department of Homeland Security said it has no credible intelligence of any threat to the United States. We will return to the Brussels attacks after the news summary.