WASHINGTON — FCC Chairman Ajit Pai said that the agency lacked authority to revoke the license of a broadcast station based on the the content of a newscast, in his first comments on the topic since President Donald Trump’s tweets last week.

Trump tweeted attacks on NBC News’ reporting and wrote, “Network news has become so partisan, distorted and fake that licenses must be challenged and, if appropriate, revoked. Not fair to public!”

Pai, appearing at an event sponsored by the Mercatus Center of George Mason University, did not directly address Trump but said that the “FCC under my leadership will stand for the First Amendment.”

He said that the agency “under the law does not have the authority to revoke the license of a broadcast station based on the content of a particular newscast.”

The FCC grants licenses not to broadcast networks, but broadcast stations. It regularly reviews the licenses, typically examining a station’s commitment to programming and activities in the public interest, but in the vast majority of cases they are mere formalities.

But Trump’s tweets generated alarm among First Amendment advocates, who said that they could have a calling effect on freedom of speech. Pai did not respond to them until the Wednesday event, as some of his fellow commissioners called on him to respond. Jessica Rosenworcel, a Democratic appointee, said on CNN’s “Reliable Sources” that “history will not be kind of silence.”

Sen. Richard Blumenthal (D-Conn.) sent a letter to Pai on Monday asking him to respond to Trump’s tweets and asking him to commit that the First Amendment “remains a cornerstone of our democracy.”

But Pai has refrained from directly criticizing Trump, who appointed him as chairman and nominated him to a new term, while asserting that he is an avid defender of the First Amendment.

After Pai’s remarks, Andrew Jay Schwartzman of Georgetown University Law Center’s Institute for Public Representation, said that “unlike his predecessors, who have forthrightly stood up to Presidential interference, he continues to equivocate. He needs to say that President Trump has has no right to interfere in the FCC’s licensing process and he will ignore the President’s pressure.”

Robert McDowell, a former commissioner who is now with the firm Cooley LLP, challenged the notion that Pai has not been outspoken on the issue.

“He hasn’t responded directly to it, but he has long been on the record about defending free speech rights of broadcasters and political speech and news.” He noted that he and Pai had won the Media Institute’s Freedom of Speech Award.

Last month, Pai gave a speech at the Newseum where he said that he talked of freedom of speech being “under siege,” particularly on college campuses. He said that he saw some of these “worrying signs” in his role at the FCC, pointing to Twitter messages in which “people regularly demand that the FCC yank licenses from cable news channels like Fox News, MSNBC, or CNN because they disagree with the opinions expressed on those networks.”

Update: Nine Democrats on the Senate Commerce Committee are calling for a hearing on Trump’s attacks on the media and the FCC response.

“It took six days and pressure from Congress and the media for FCC Chairman Ajit Pai to respond and Commissioners Michael O’Rielly and Brendan Carr continue to remain silent on the President’s threats,” the senators wrote in a letter to Senate Commerce Committee Chairman John Thune (R-S.D.) and its ranking member, Sen. Bill Nelson (D-Fla.). “This silence is inexcusable on an issue that goes not only to the independence of the FCC, but also to the core of our democracy and the freedoms we value as Americans.”