President-elect Donald Trump went on the attack Thursday, a day ahead of his promised briefing on why U.S. intelligence agencies are convinced Russia was behind election-related hacking, questioning how the conclusions could be reached without examining the servers at the Democratic National Committee.

Trump also asked how NBC News got its hands on the top-secret report given to President Barack Obama Thursday. NBC, however, merely confirmed Obama had been briefed.

BuzzFeed News reported Wednesday the FBI never requested access to the DNC servers after WikiLeaks started publishing emails that proved embarrassing to the party right before its national nominating convention in July.

“The DNC had several meetings with representatives of the FBI’s Cyber Division and its Washington Field Office, the Department of Justice’s National Security Division and U.S. Attorney’s Offices, and it responded to a variety of requests for cooperation, but the FBI never requested access to the DNC’s computer servers,” Eric Walker, the DNC’s deputy communications director, told BuzzFeed in an email.

Trump has expressed skepticism Russia was behind the hacking, taking the word of WikiLeaks founder Julian Assange that the DNC emails and emails from the hacked Gmail account of Clinton campaign Chairman John Podesta came from another source.

Director of National Intelligence James Clapper testified before the Senate Armed Services Committee Thursday, saying Russia had stepped up its cyberespionage efforts and criticized Trump for “disparaging” U.S. intelligence services.

Trump was scheduled to be briefed Friday. An unclassified version of the report was expected to be released to the public next week.

Trump, however, refused to wait for the official briefing, lashing out in a series of tweets.

Clapper testified Russian President Vladimir Putin ordered the hacking and said the motive would be released next week. Russia has denied involvement, but Obama said at a news conference last month very little goes on in Russia without Putin’s approval.

"Russia is a full-scope cyber-actor that poses a major threat to US government, military, diplomatic, commercial and critical infrastructure," intelligence officials said in prepared joint testimony.

The Department of Homeland Security released a declassified report last month that concluded Russia tried to bolster Trump’s candidacy.

The U.S. last week imposed new sanctions on Moscow, closing two U.S. compounds and expelling 35 diplomats.