A gathering in Brussels to remember the victims of the terrorist attacks. (Valentina Media/Flickr)
They killed 31 people and injured about 260 others in two locations in Brussels, Belgium: the airport and a city metro station.
Who are these men and how did they manage to carry off such a major terrorist attack?
At least one was already suspected of terrorism
Brothers Khalid El Bakraoui, age 27, and Ibrahim El Bakraoui, age 30, were two of the suicide bombers. Khalid carried out the Maelbeek metro bombing that killed at least 20 people, while Ibrahim was part of the Zaventem airport bombing that killed another 11.
Turkey had already suspected one brother of being a terrorist, but Belgium wasn't watching him or his brother.
There are close links between Brussels and Paris
One of the men who apparently carried out the bombing at the Brussels airport was Najim Laachraoui, who was suspected of being a bomb maker who helped carry out the Paris terrorist attacks last November.
On top of that, the El Bakraoui brothers had helped the Paris bombers by helping them get weaponry and providing them with a safe house in Belgium to meet before they carried out that attack.
And the mastermind of the Paris attack, Saleh Abdeslam, was arrested days before the Brussels attack. If he hadn't been, he apparently would have helped carry out this attack too. (Abdeslam was born in Belgium.)
Right after Paris happened, given the bombers' links to Belgium, there were immediate fears about a similar attack in Brussels. Those fears, horribly, came true.
They had materials for way, way more bombs
As bad as the Brussels bombing was, it clearly could have been much worse. Because the bombers had a couple of bombs and a suicide belt at the airport that didn't go off, and had left other bombs in suitcases at their house.
Those bombs had tons of this bomb-making material called TATP. Well, not literally tons--pounds--but it's seriously a lot.
It's scarily easy to make TATP, which is why it's hard for authorities to figure out who's making it.
Not 1, but 2 bombers are still on the run
Three bombers died in the Brussels attacks, and police realized immediately that one had gotten away. Three dead bombers, another one out there somewhere. And then they discovered there was a 5th terrorist.
It's not 100% clear, but it seems that one terrorist from each attack location got away: one from the airport, the other from the metro. The airport bomber is being called the man in white because he was caught on camera wearing a white shirt and a tan jacket.
The 3 Brussels bombing suspects. The two on the left are the brothers. (Wikimedia Commons)
There's an intense manhunt going on right now for these two.
There are many more terrorists like these
Experts believe hundreds of ISIS terrorists have been trained and sent throughout Europe to carry out more attacks like the ones in Paris and Brussels.
It's relatively easy for terrorists to move around Europe, and relatively hard for intelligence to track them. Plus, European countries aren't sharing all the intel they are gathering.
These aren't just insurgents coming from countries where ISIS has taken hold, like Syria and Iraq. Many of them are European themselves--often "third-generation Muslim immigrants, who have become radicalized in poor communities left to develop outside the national culture."
So while several presidential candidates in the US are focusing on keeping refugees out due to terrorism fears, Europe is struggling with homegrown terrorists.
This article was written by Holly Epstein Ojalvo and originally appeared on Kicker. Kicker explains the most important, compelling things going on in the world and empowers you to get in the know, make up your own mind, and take action. For more, check out the Kicker site, like their Facebook page, or subscribe to their email newsletter.