DENVER, Colo. -- The terror attacks in Brussels Tuesday may come as a surprise to some, but the intelligence community has anticipated impending attacks in Belgium since the country was linked to the Paris killings back in November.
The Islamic State claimed responsibility for the Brussels attacks that killed at least 30 people and injured more than 200 others.
"It's very clear, a lot of pressure has been put on ISIS, and even though I think ISIS has been pushed back, I think that's the time it becomes even more dangerous," said former U.S. Ambassador Christopher Hill, who is now the Dean of the Josef Korbel School of International Studies at the University of Denver. "They want to signal to all their supporters around the world that they're alive and well."
It's still not clear if Friday's arrest of French terror suspect Salah Abdeslam in a Brussels suburb sparked Tuesday's violence in some way.
"Usually, these attacks are ones that have had a considerable amount of planning, casing of the places where they're going to hit," said Hill. "I suspect this has been going on long before the recent arrest of the last known Paris attackers."
Belgian police have been following terror cells for months. Right after the Paris attacks, Brussels was on a four-day lockdown. Just days later, police raided a small town south of the city, looking for accomplices.
Authorities have also been keeping an eye on Molenbeek, a largely Muslim suburb east of Brussels. Forty percent of the 95,000 people who live there are of the Islamic faith. Extremists linked to November Paris attacks, the Charlie Hebdo shootings, the Brussels Jewish museum shootings and Paris-Amsterdam train shootings all spent time in Molenbeek.
On New Year's Eve, a terror alert canceled the celebratory fireworks, keeping the 100,000 people who usually fill the streets at home.
"To understand Brussels is to understand a city that's not just the capital of the kingdom of Belgium, it's really the capital of Europe," said Hill. "It is a city which has an enormous number of people going in and out on a daily basis. It is truly an international city."
Clearly, Brussels has also become a target for extremism.
Police in Belgium and France have arrested nearly 30 people, they say, were involved in the Paris attacks that killed 130 people.