Presidential candidates from both parties decried the bombings at Brussels' Zaventem Airport and Metro stations, with Donald Trump, Ted Cruz and Hillary Clinton all directing blame toward Muslim attackers for the bloodshed.
Republican frontrunner Donald Trump warned that the attacks in Brussels, which killed more than 34 people and wounded 187 more, "is just the beginning." He also repeated his call for increased monitoring of who enters the United States. During his campaign, Trump has advocated barring Muslims from entering the US.
"It's going to get worse and worse. In my opinion, this is just the beginning," Trump told CNN. "It will get worse and worse because we are lax and we are foolish -- we can't allow these people, at this point we cannot allow these people to come into our country. I'm sorry. This is a story that just seems to be more and more happening and it's really not very pretty to watch."
Trump also repeated his openness to waterboarding terror suspects. Waterboarding is a torture tactic used by the US following the attacks of September 11, 2001, yet it is considered inhumane and illegal by international law standards and ineffective by many law enforcement and military experts. The US has since banned its use.
"You know, we work within laws. They don't work within laws -- they have no laws. We work within laws. The waterboarding would be fine, and if they could expand the laws, I would do a lot more than waterboarding," he said.
Ripping "liberal policies" in the US, he said terror attacks like those in Brussels are not "our fault."
"We're not the victims here -- we're acting like this is our fault," he said. "That's the problem with the liberal policies of this country and this world, it's acting like it's our fault. It's not our fault, okay, it's not our fault. It's their fault. And they have to come out and they have to say, hey look, this is happening."
Trump told NBC's 'Today Show' that "they are not assimilating for whatever reason.
"They don't want laws that we have, they want Sharia Law, and you say to yourself, at what point, how much of this do you take?"
Democratic frontrunner Hillary Clinton said in a statement that "the world will not be intimidated by these vicious killers."
"These terrorist seek to undermine the democratic values that are the foundation of our alliance and our way of life, but they will never succeed," she said. "Today's attacks will only strengthen our resolve to stand together as allies and defeat terrorism and radical jihadism around the world."
Clinton told the 'Today Show' that closing US borders was "unrealistic."
"It's unrealistic to say we're going to completely shut down our borders to everyone," she said. "I know that Americans have every reason to be frightened by what they see, (but) we've got to work this through, consistent with our values."
She also said she opposed waterboarding because US military leaders have said it's not effective.
Bernie Sanders, Clinton's rival in Democratic race, tweeted his condolences and called the bombings a "barbaric attack."
Senator Ted Cruz (R-Texas) said "these terror attacks are no isolated incidents."
"They are just the latest in a string of coordinated attacks by radical Islamic terrorists perpetrated by those who are waging war against all who do not accept their extreme strain of Islam," he said in a Facebook statement.
He chastised President Barack Obama, saying the president "refuses to acknowledge" that "radical Islam is at war with us."
He said when he's elected president, "we will defeat" radical Islam.
In a statement, Governor John Kasich (R-Ohio) said: "We must utterly reject the use of deadly acts of terror."
"We must also redouble our efforts with our allies to identify, root out and destroy the perpetrators of such acts of evil," Kasich said. "We must strengthen our alliances as our way of life and the international system that has been built on our common values since the end of the Second World War comes under challenge from these and other actors of evil."