“If this story is accurate, that means someone leaked the President’s briefing papers. Leaking such information is a fireable offense and likely illegal,” a senior White House official said in a statement.

The Washington Post reported late Tuesday that a section of Trump’s briefing materials for his call with Putin read “DO NOT CONGRATULATE” in all-capital letters.

Trump also did not follow instructions to condemn Putin over the poisoning of a former Russian double agent and his daughter in the U.K., according to the Post.

The White House has not confirmed the report, but multiple news outlets followed with similar stories. The person or people responsible for the leak have not been publicly identified.

It’s not clear whether Trump received the warning before getting on the call with Putin. The Post reported that national security adviser H.R. McMaster briefed the president in person about the call, but did not mention the issue.

It renewed questions about Trump’s approach toward Russia, which is frequently at odds with U.S. allies, Congress and some members of his own administration.

Instead of confronting Putin about a host of tense issues, Trump not only congratulated the Russian leader but also discussed a possible summit meeting with him.

The fact that sensitive briefing materials leaked out also alarmed top administration officials , who are concerned members of Trump’s own staff could be trying to undermine him.