WASHINGTON — American law enforcement and intelligence agencies are examining intercepted communications and financial transactions as part of a broad investigation into possible links between Russian officials and associates of President-elect Donald J. Trump, including his former campaign chairman Paul Manafort, current and former senior American officials said.

The continuing counterintelligence investigation means that Mr. Trump will take the oath of office on Friday with his associates under investigation and after the intelligence agencies concluded that the Russian government had worked to help elect him. As president, Mr. Trump will oversee those agencies and have the authority to redirect or stop at least some of these efforts.

It is not clear whether the intercepted communications had anything to do with Mr. Trump’s campaign, or Mr. Trump himself. It is also unclear whether the inquiry has anything to do with an investigation into the hacking of the Democratic National Committee’s computers and other attempts to disrupt the elections in November. The American government has concluded that the Russian government was responsible for a broad computer hacking campaign, including the operation against the D.N.C.

The counterintelligence investigation centers at least in part on the business dealings that some of the president-elect’s past and present advisers have had with Russia. Mr. Manafort has done business in Ukraine and Russia. Some of his contacts there were under surveillance by the National Security Agency for suspected links to Russia’s Federal Security Service, one of the officials said.

Mr. Manafort is among at least three Trump campaign advisers whose possible links to Russia are under scrutiny. Two others are Carter Page, a businessman and former foreign policy adviser to the campaign, and Roger Stone, a longtime Republican operative.

The F.B.I. is leading the investigations, aided by the National Security Agency, the C.I.A. and the Treasury Department’s financial crimes unit. The investigators have accelerated their efforts in recent weeks but have found no conclusive evidence of wrongdoing, the officials said. One official said intelligence reports based on some of the wiretapped communications had been provided to the White House.

Counterintelligence investigations examine the connections between American citizens and foreign governments. Those connections can involve efforts to steal state or corporate secrets, curry favor with American government leaders or influence policy. It is unclear which Russian officials are under investigation, or what particular conversations caught the attention of American eavesdroppers. The legal standard for opening these investigations is low, and prosecutions are rare.

“We have absolutely no knowledge of any investigation or even a basis for such an investigation,” said Hope Hicks, a spokeswoman for the Trump transition.

In an emailed statement Thursday evening, Mr. Manafort called allegations that he had interactions with the Russian government a “Democrat Party dirty trick and completely false.”

“I have never had any relationship with the Russian government or any Russian officials. I was never in contact with anyone, or directed anyone to be in contact with anyone,” he said.

“On the ‘Russian hacking of the D.N.C.,’” he said, “my only knowledge of it is what I have read in the papers.”

The decision to open the investigations was not based on a dossier of salacious, uncorroborated allegations that were compiled by a former British spy working for a Washington research firm. The F.B.I. is also examining the allegations in that dossier, and a summary of its contents was provided to Mr. Trump earlier this month.

Representatives of the agencies involved declined to comment. Of the half-dozen current and former officials who confirmed the existence of the investigations, some said they were providing information because they feared the new administration would obstruct their efforts. All spoke on condition of anonymity because they were not authorized to discuss the cases.

Numerous news outlets, including The New York Times, have reported on the F.B.I. investigations into Mr. Trump’s advisers. On Wednesday, McClatchy revealed the existence of a multiagency working group to coordinate investigations across the government.

The continuing investigation again puts the F.B.I. director, James B. Comey, in the middle of a politically fraught investigation. Democrats have sharply criticized Mr. Comey’s handling of the investigation into Hillary Clinton’s use of a private email server. Mrs. Clinton has said his decision to reveal the existence of new emails late in the campaign cost her the election.

The F.B.I. investigation into Mr. Manafort began last spring, and was an outgrowth of a criminal investigation into his work for a pro-Russian political party in Ukraine and for the country’s former president, Viktor F. Yanukovych. In August, The Times reported that Mr. Manafort’s name had surfaced in a secret ledger that showed he had been paid millions in undisclosed cash payments. The Associated Press has reported that his work for Ukraine included a secret lobbying effort in Washington aimed at influencing American news organizations and government officials.

Mr. Stone, a longtime friend of Mr. Trump’s, said in a speech in Florida last summer that he had communicated with Julian Assange, the founder of WikiLeaks, the anti-secrecy group that published the hacked Democratic emails. During the speech, Mr. Stone predicted further leaks of documents, a prediction that came true within weeks.

In a brief interview on Thursday, Mr. Stone said he had never visited Russia and had no Russian clients. He said that he had worked in Ukraine for a pro-Western party, but that any assertion that he had ties to Russian intelligence was “nonsense” and “totally false.”

“The whole thing is a canard,” he said. “I have no Russian influences.”

The Senate intelligence committee has started its own investigation into Russia’s purported attempts to disrupt the election. The committee’s inquiry is broad, and will include an examination of Russian hacking and possible ties between people associated with Mr. Trump’s campaign and Russia.

Investigators are also scrutinizing people on the periphery of Mr. Trump’s campaign, such as Mr. Page, a former Merrill Lynch banker who founded Global Energy Capital, an investment firm in New York that has done business with Russia.

In an interview,, Mr. Page expressed bewilderment about why he might be under investigation. He blamed a smear campaign that he said was orchestrated by Mrs. Clinton for the news media speculation about the nature of his ties to Russia.

“I did nothing wrong, for the 5,000th time,” he said. His adversaries, he added, are “pulling a page out of the Watergate playbook.”

The lingering investigations will pose a test for Senator Jeff Sessions, Republican of Alabama, who has been nominated for attorney general. If Mr. Sessions is confirmed, he will for a time be the only person in the government authorized to seek foreign intelligence wiretaps on American soil.

Mr. Sessions said at his confirmation hearing that he would recuse himself from any investigations involving Mrs. Clinton. He was not asked whether he would do so in cases involving associates of Mr. Trump.