JUDY WOODRUFF: The city of Brussels began three days of mourning today; 31 people were killed and 270 others wounded in Tuesday's suicide attacks.

Today, investigators kept up a manhunt, as the city tried to get back to something like normal.

Malcolm Brabant reports from Brussels.

MALCOLM BRABANT: Hundreds gathered in Brussels' Place de la Bourse this morning to remember the victims of yesterday's twin bombings with a moment of silence.

WOMAN: Well, I think everyone can agree that this is a horrible, horrible, horrible thing. I think that's all there is left to say about it.

CAROLINE LEDENT, Brussels Resident (through interpreter): I think we had to be here. I don't have the words to express what I feel, but it's true that we have been used to see these attacks for a long time, but when it happens at home, you feel it in a stronger way.

MALCOLM BRABANT: Elsewhere in the city, flags at European Union buildings were lowered to half-staff. European leaders laid a wreath at the metro station where one of the bombers struck, and Belgium's king and queen visited with staff and emergency responders at the Brussels Airport, which announced it will remain closed at least another day.

Many others in the Belgian capital took pride in returning to work and routine as a sign of strength in the face of terror.

WOMAN: We all have to, because, otherwise, I think they will win. That's what they want, to paralyze our lives in Europe, and we have to go on with it.

WOMAN: We shouldn't be afraid. I think the most straight answer is to be there, to continue our lives, to go to the terraces, to drink our coffee, to go to the cinema and so on.

MALCOLM BRABANT: But, as life went on, so did the search for answers. Investigators now believe at least four individuals were involved in the attacks.

They say Ibrahim El Bakraoui and another man seen were the suicide bombers who struck the departures concourse. That second man, Najim Laachraoui, allegedly also made the suicide vests for the Paris bombings.

And it was El Bakraoui's brother, Khalid, who blew himself up on a subway car near the European Union complex about an hour later. The third man, not yet identified from the airport photo, is still being sought.

FREDERIC VAN LEEUW, Belgian Federal Prosecutor (through interpreter): The third suspect wearing a light jacket and a hat is on the run. He put down a large bag, then left before the explosion. It contained the largest explosive charge. Shortly after the arrival of the bomb disposal unit, this bag blew up because of the highly unstable nature of the explosives. Fortunately, nobody was injured.

MALCOLM BRABANT: Officials say the two brothers did have criminal records tied to robbery and carjacking, but it wasn't until a March 15 raid on a Belgium apartment rented by Khalid El Bakraoui that they became suspects in the ongoing terror probe.

That search also turned up fingerprints belonging to Salah Abdeslam, the top suspect in November's terror attacks in Paris. He was captured last week. One of several raids since yesterday's attacks turned up explosives and chemicals used to make bombs, as well as Ibrahim El Bakraoui's computer on which he said he was unsure what to do, hunted everywhere, and no longer safe.

Today, Turkish President Recep Tayyip Erdogan said one of the attackers was deported from Turkey in June.

PRESIDENT RECEP TAYYIP ERDOGAN, Turkey (through interpreter): We informed the Belgium Embassy with a diplomatic note about the deportation on July 14, 2015. Despite our warnings that this person is a foreign fighter, Belgium could not establish any links with terrorism.

MALCOLM BRABANT: Likewise, French officials have complained that Belgium failed to conduct security crackdowns in Muslim areas after the Paris attacks. French Prime Minister Manuel Valls called for greater coordination today, before traveling to Brussels.

MANUEL VALLS, Prime Minister, France (through interpreter): We closed our eyes everywhere in Europe, including France, to the progression of extremist ideas, neighborhoods which through a combination of drug trafficking and radical Islamism perverted youth. And I'm not here to lecture the Belgians, because I'm sure they're more than aware of this.

MALCOLM BRABANT: There was also word that Secretary of State John Kerry will travel to Brussels on Friday to meet with top Belgian and European officials.

And, tonight, European and Iraqi intelligence officials told the Associated Press that ISIS has dispatched at least 400 fighters to Europe, and they have been specifically trained to attack the West — Judy.

JUDY WOODRUFF: So, Malcolm, given the threat of these fighters, give us a sense of the security across Europe. You traveled into Belgium today.

MALCOLM BRABANT: Yes, I have come from Lesbos in Greece, and have traveled through Athens and Paris.

And all the way along that route, you can see just how sensitive people are in the wake of these attacks at transport hubs. At Athens Airport, for example, today, there was an alarm going off, and you could see the tension in people's faces because they just didn't know what was going on.

In Paris, the plane was met by policemen checking very assiduously everybody's passports. And this is something that shouldn't normally happen within the Schengen free travel zone. This is supposed to be paper-free. But they were checking everybody's identities very closely.

Indeed, there were sniffer dogs there. And then driving into Belgium wasn't a problem. The border was wide open, but on the other side of the motorway, it was very difficult getting out. There were hordes of policemen on the border with France trying to see if they could get hold of those people that might be involved in those attacks yesterday.

And when I finally got into the center of Belgium, into Brussels this morning — this afternoon, rather, at the Eurostar station, it looked very much like this was a city at war. There were army trucks everywhere, police guards, and people checking — being checked as they went into the Eurostar station, because this is something that people really are concerned about, that, in order to get into these places now, you perhaps need to be checked.

And then there was a queue outside. And these are the sort of things that can become soft targets in the future. So, there's a real difficulty about the balance of security in places like Eurostar, train station and at airports.

JUDY WOODRUFF: And just quickly, Malcolm, you reported on Turkey having deported back to Belgium one of the attackers from what happened yesterday. How do Belgians explain this?

MALCOLM BRABANT: Well, this is really embarrassing for the Belgians, because the Turkish president said that the Belgian authorities were warned that this man did have terrorist links, and he said that the Belgians had not been able to find any.

And yet this man was able to wander around in Belgium for months before carrying out this terrible suicide attack. And it really does beg the question about what the European authorities are going to do about people who return from Syria, especially as this is where they're getting the military expertise to be able to carry out the sort of attacks that we saw in Brussels yesterday.

JUDY WOODRUFF: Malcolm Brabant reporting for us tonight from Brussels, we thank you.

And we will take a closer look at Europe's struggle to prevent this sort of attack after the news summary.