WASHINGTON — The nation’s top intelligence officials are making their most detailed and persuasive case yet to President-elect Donald Trump that Russia interfered in this year’s U.S. political process.

The officials — Director of National Intelligence James Clapper, CIA Director John Brennan and FBI Director James Comey — are preparing to point to multiple motives for Moscow’s alleged meddling as they brief Trump on their classified report Friday in New York. President Barack Obama received a briefing on Thursday, and a declassified version of the report is expected to be released at some point.

Since winning the election, Trump has repeatedly questioned intelligence officials’ assessments that Russia was behind the hacking of the Democratic National Committee and individual Democrats like Hillary Clinton campaign chairman John Podesta.

Trump remained dubious about the assertion even on the eve of his intelligence briefing, asking how officials could be “so sure” about the hacking if they had not examined DNC servers.

“What is going on?” he wrote on Twitter.

A senior law enforcement official said the FBI repeatedly stressed to DNC officials the importance of obtaining direct access to the servers “only to be rebuffed until well after the initial compromise had been mitigated.” The official said the FBI had to rely on a “third party” for information, but did get access to the material it needed.

The Washington Post, citing anonymous U.S. officials, reported Thursday that intelligence agencies have identified parties who delivered stolen Democratic emails to WikiLeaks. The officials also said there were disparities between efforts to infiltrate Democratic and Republican networks, and said the U.S. intercepted communications in which Russian officials celebrated Trump’s victory. It was not clear which of those details were included in the classified report.