WASHINGTON - The FBI is investigating whether Donald Trump's associates coordinated with Russian officials in an effort to sway the 2016 presidential election, Director James Comey said Monday in an extraordinary public confirmation of a probe the president has refused to acknowledge, dismissed as fake news and blamed on Democrats.

In a bruising five-hour session, the FBI director also knocked down Trump's claim that his predecessor had wiretapped his New York skyscraper, an assertion that has distracted White House officials and frustrated fellow Republicans who acknowledge they've seen no evidence to support it.

The revelation of the investigation of possible collusion with Russians, and the first public confirmation of the wider probe that began last summer, came in a remarkable hearing by one branch of government examining serious allegations against another branch and the new president's election campaign.

Tight-lipped for the most part, Comey refused to offer details on the scope, targets or timeline for the FBI investigation, which could shadow the White House for months, if not years. The director would not say whether the probe has turned up evidence that Trump associates may have schemed with Russians during a campaign marked by email hacking that investigators believe was aimed at helping the Republican defeat Democrat Hillary Clinton.

"I can promise you," the FBI director vowed, "we will follow the facts wherever they lead."

Comey for the first time put himself publicly at odds with the president by contradicting a series of recent tweets from Trump that asserted his phones had been ordered tapped by President Barack Obama during the campaign.

"With respect to the president's tweets about alleged wiretapping directed at him by the prior administration, I have no information that supports those tweets, and we have looked carefully inside the FBI," Comey said. The same was true, he added, of the Justice Department.

His confirmation of the Russia-links investigation was striking given the FBI's historic reluctance to discuss its work. But Comey said the intense public interest in the matter - and permission from the Justice Department - made it appropriate to do so.

Comey said the collusion inquiry began last July as part of a broader probe into Russian meddling in American politics, meaning Trump was elected president as associates remained under investigation for possible connections to Russia.

Clinton allies on Monday contrasted Comey's silence during the campaign with public comments he made last year when closing out an investigation into Clinton's email practices and then, shortly before Election Day, announcing that the probe would be revived following the discovery of additional emails. Many Democrats blame Comey's public updates with stoking worries about Clinton's trustworthiness and turning voters against her.

Comey acknowledged that "some folks may want to make comparisons to past instances" where he and other officials were more open, but he said those were about concluded investigations.

In the current case, it's not clear how long it will take for the FBI to decide if a crime was committed, but counterintelligence investigations are known for being complicated and time-intensive - and for frequently concluding without charges. Comey would not commit to a timetable.

Regardless of the outcome, the investigation is unquestionably an unwelcome distraction for an administration that has struggled to move past questions about ties to Russia. The White House tried anew Monday to distance itself from two former senior members of Trump's team, Paul Manafort and Michael Flynn, who have been under scrutiny for foreign contacts.

Rep. Devin Nunes, the California Republican, told Comey that revelations about the investigation had placed a "big gray cloud" over people trying to lead the country.

"The faster you can get to the bottom of this, it's going to be better for all Americans," he said.

The hearing quickly divided along partisan lines. Democrats pressed for details on the status of the FBI's investigation, while Republicans focused on news coverage and possible improper disclosures of classified information developed through surveillance.

Comey is the latest government official to reject Trump's claims, made without any evidence, that Obama had wiretapped Trump Tower, his campaign headquarters. Rep. Nunes rejected them earlier in the hearing.

Comey testified along with National Security Agency Director Michael Rogers, who also disputed allegations that British intelligence services could have been involved in such wiretapping. The White House last week pointed to a report of British involvement in an attempt to bolster the president's claim. The move only angered an ally.

Trump took to Twitter before Monday's hearing began, accusing Democrats of making up allegations about his campaign associates. He said Congress and the FBI should be going after media leaks and maybe even Clinton instead.

"The real story that Congress, the FBI and others should be looking into is the leaking of Classified information. Must find leaker now!" Trump tweeted early Monday as news coverage on the Russia allegations dominated the morning's cable news.

The president continued to tweet throughout the hearing, creating a unusual public conversation between the embattled president and his FBI director.

After Trump tweeted that the FBI and NSA had told Congress that Russia did not influence the electoral process, Comey disputed that description. The FBI has offered no opinion and has no view and no information on the potential impact on the election because that's not something the bureau has looked at, he said.

The president also claimed that Comey had said there was no evidence of collusion between his aides and Russia, though Comey said no such thing.

Trump also suggested, without evidence, that Clinton's campaign was in contact with Russia and had possibly thwarted a federal investigation. U.S. intelligence officials have not publicly raised the possibility of contacts between the Clintons and Moscow. Officials have said they believe Moscow had hacked into Democrats' computers in a bid to help Trump's election bid.

The panel's ranking Democrat, Rep. Adam Schiff of California, outlined a chronology that he said suggested frequent and troubling contacts between Trump associates and Russian intermediaries.

"Is it possible that all of these events and reports are completely unrelated and nothing more than a entirely unhappy coincidence?" he asked rhetorically. "Yes, it is possible. But it is also possible, maybe more than possible, that they are not coincidental, not disconnected and not unrelated."