President Donald Trump continued challenging big tech companies on Tuesday, warning them about censoring content from conservatives.

“I think what Google and what others are doing – if you look at what is going on with Twitter and if you look at what’s going on in Facebook, they better be careful because you can’t do that to people,” Trump said. “You can’t do it.”

The president said that he had heard “thousands” of censorship complaints from people and repeated that it was “not fair.” He added that censorship was a “serious charge” against the tech companies.

“I think that Google and Twitter and Facebook, they are really treading on very, very troubled territory and they have to be careful,” he said. “It is not fair to large portions of the population.”

The president has recently ramped up his criticisms of the social media companies after reports of conservative content being downgraded, removed, or deleted from their platforms.

Axios reporter Jonathan Swan suggested on Twitter that the president was reacting to a series of Breitbart News stories on the biased search results and censorship by tech companies.

On Tuesday morning, the president commented on a report that argued Google was favoring liberal and establishment media sources on their Google News searches and burying conservative content.