() Former New York City Mayor Bill de Blasio calls the detention of Columbia University activist Mahmoud Khalil “deeply troubling.”

Khalil is in Immigration and Customs Enforcement custody as federal attorneys argue for his deportation. Khalil, a permanent U.S. resident who holds a green card, was part of pro-Palestinian rallies on the school’s campus last year.

The government has accused Khalil of engaging in pro-Hamas activities. De Blasio joined to discuss the case, which the ACLU has called one of the biggest threats to free speech in 50 years.

“I have heard allegations that this was someone who passed out pro-Hamas leaflets, except, how come we haven’t seen any pictures of these leaflets? How come we haven’t seen any evidence?” de Blasio asked. “Is the next step going to be American citizens start to get arrested on hearsay because of something they said? That’s a dangerous, dangerous precedent.”

De Blasio said he supports Israel but also believes people have a right to disagree with the policies of Israel and nobody should be arrested for exercising free speech and disagreeing with a government’s policies.

“I’ve not seen any evidence of him being ‘pro-Hamas.’ I’ve seen evidence that he wanted to see a different outcome in Gaza,” de Blasio told .

Secretary of State Marco Rubio has argued that Khalil’s detention isn’t a free speech issue, saying that nobody has a right to a student visa or a green card and that America would not let someone in who intended to engage in protests.

De Blasio said Rubio is incorrect.

“The Supreme Court has said if you have a green card, you’re on the path to citizenship. You have the rights of an American,” he said. “You don’t get arrested for disagreeing with the U.S. government.’

With no evidence that Khalil was encouraging specific acts of violence, de Blasio said Americans on both sides of the aisle should be worried about what the case means for free speech.

“He was working with the Columbia administration, trying to come up with some kind of agreement to end the protest. How does someone like that end up in jail with no freedom and charges against him when he has a green card?” de Blasio said. “We should really be concerned about this one.”