WASHINGTON - The Democratic National Committee rebuffed requests by federal agents to inspect computer servers that had been breached last year during the presidential campaign, forcing them to rely on third-party cybersecurity data to investigate the hack, the FBI said.

The revelation came hours before U.S. intelligence chiefs are set to brief President-elect Donald Trump on their assessment that Russia was behind the attack. On Capitol Hill Thursday, they rejected Trump’s repeated skepticism about their findings that senior Russian officials were to blame for the hacking and leaks of emails from Democratic officials and organizations backing Hillary Clinton.

After the hearing, Trump took to Twitter to again express his doubts about their conclusions.

“The Democratic National Committee would not allow the FBI to study or see its computer info after it was supposedly hacked by Russia,” Trump said. “So how and why are they so sure about hacking if they never even requested an examination of the computer servers? What is going on?”

The FBI on Thursday released a statement confirming Trump’s claim.

“The FBI repeatedly stressed to DNC officials the necessity of obtaining direct access to servers and data, only to be rebuffed until well after the initial compromise had been mitigated,” the agency said. “This left the FBI no choice but to rely upon a third party for information. These actions caused significant delays and inhibited the FBI from addressing the intrusion earlier.”

Director of National Intelligence James Clapper, CIA Director John Brennan and FBI Director James Comey are scheduled to brief Trump Friday.

While intelligence agencies hadn’t previously confirmed that the DNC refused to provide access to its computers, they had disclosed that they depended on private cyber security companies.

Calls to the DNC press office were not immediately returned.

In October, Timothy Barrett, spokesman for the Office of the Director of National Intelligence, said that U.S. intelligence “independently observed technical activity that is consistent with the forensic evidence identified by a private cyber-firm and is consistent with our general understanding of cyber activities by the Russian government.”

Although Barrett didn’t name the company, CrowdStrike Inc. released technical details last year to demonstrate hacking attacks against the DNC and other groups were carried out by the Russian government.

CrowdStrike said it found evidence that hacking attacks were carried out by two Russian government hacking groups. One that it calls Fancy Bear is believed to be an arm of Russia’s military intelligence agency. The other, which it calls Cozy Bear, is believed to be run by Russia’s Federal Security Service, the successor to Russia’s KGB, where Russian President Vladimir Putin once worked.

Clapper Thursday told the Senate Armed Services Committee that the intelligence agencies’ confidence in their findings is now “very high” and they are “even more resolute” about Russian involvement than when they first weighed in on the issue publicly on Oct. 7. Yet Clapper also emphasized that Russian hacking didn’t change the vote count that made Trump president.

President Obama last week imposed sanctions against top Russian intelligence officials and agencies and expelled 35 Russian operatives from the U.S. Russia has denied any role in the computer attack. Putin has vowed “a proportional” response, though is holding off until after Trump takes office Jan. 20.