“I would close up our borders to people until we figure out what’s going on,” Trump said Tuesday morning on Fox News. “We have to be smart in the United States. We’re taking in people without real documentation, we don’t know where they’re coming from, we don’t know what they’re — where they’re from, who they are.”

During an appearance on NBC’s “Today” show, Trump reiterated his commitment to using waterboarding to gain information from terror suspects in custody.

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“Frankly, the waterboarding, if it was up to me, and if we changed the laws or had the laws, waterboarding would be fine,” Trump told Savannah Guthrie and Matt Lauer when asked about what techniques he favored. “If they could expand the laws, I would do a lot more than waterboarding. You have to get the information from these people.”

Experts across the political spectrum harshly criticized Trump’s statements.

Appearing on Fox News to discuss the attacks in Belgium, Michael Chertoff, who was the secretary of homeland security in the George W. Bush administration, labeled the candidate’s ideas “preposterous.”

“First of all, we have a much, much tougher refugee program than the Europeans have,” Chertoff said. “The problem the Europeans have is people showed up on their doorstep — hundreds of thousands, coming directly from the region. That does not happen in the U.S. We check people very carefully before we admit them as refugees.”

Chertoff — who served as secretary from 2005 to 2009 — added that it’s not impossible for someone to slip into the United States and carry out a major terror attack. But, he said, improvements in the country’s visa waiver program and officials’ ability to review data make it “much much harder in the U.S.”

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“On the other hand, if you look historically back at people who carried out terrorist attacks in the U.S., many of them didn’t start out as Muslims,” Chertoff said.

“The idea that you can identify people who are a risk based upon their religion or the way they look is completely fallacious. It’s like going after cancer with a meat axe instead of a scalpel.”

During an appearance on MSNBC, terrorism expert Malcolm Nance said Trump’s “bluster” in the wake of Tuesday’s attacks was hampering U.S. intelligence and the armed forces.

“Donald Trump right now is validating the cartoonish view that they [Islamist militant groups] tell their operatives and that they tell their terrorists,” Nance said. “That the United States is a racist nation, xenophobic, anti-Muslim, and that that’s why you must carry out terrorist attacks against them.”

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Nance said that type of rhetoric is detrimental to global counterterrorism missions.

“There are intelligence officers right now that are going to have to contend with their partners over what’s being said during the U.S. presidential race,” he told MSNBC. “It’s irresponsible, and it needs to stop.”