Christopher Hill, a University of Denver professor and former Ambassador to Iraq, believes the attacks in Brussels, Belgium were preplanned, even after the arrest on the main fugitive in November's Paris terror attack.

Salah Abdesalam was taken into custody Friday in a raid on an apartment in a Brussels suburb. Even though it seems like the attack was a reaction to the raid, Hill says that this type of attack is something that ISIS likely had been planning for awhile.

"Obviously right now we don't know, but in the past this type of attack takes planning, checking out the locations," Hill said. " I doubt very much that this was any sort of knee-jerk reaction to the events in the last couple of days."

Hill told 9NEWS that Belgium was likely a target because it is the "nerve center" of the European Union. The airport that was attacked is one of Europe's busiest.

Hill says law enforcement agencies throughout Europe and the United States have been on heightened watch. They have been "cracking down" on people with connections to ISIS. This, in turn, could make terrorists feel more threatened and plan more dangerous attacks.

"I think what we're seeing really is they are on their heels," Hill said. " But they're also even more dangerous. So we'll need to look at a lot of things."

Among those things, Hill says, is where the money for ISIS is coming from.