What caused the Barack Obama administration to begin investigating the Donald Trump campaign last summer has come into clearer focus following a string of congressional hearings on Russian interference in the presidential election.

It was then-CIA Director John O. Brennan, a close confidant of Mr. Obama’s, who provided the information — what he termed the “basis” — for the FBI to start the counterintelligence investigation last summer. Mr. Brennan served on the former president’s 2008 presidential campaign and in his White House.

Mr. Brennan told the House Intelligence Committee on May 23 that the intelligence community was picking up tidbits on Trump associates making contacts with Russians. Mr. Brennan did not name either the Russians or the Trump people. He indicated he did not know what was said.

But he said he believed the contacts were numerous enough to alert the FBI, which began its probe into Trump associates that same July, according to previous congressional testimony from then-FBI director James B. Comey.

The FBI probe of contacts came the same month the intelligence community fingered Russian agents as orchestrating hacks into Democratic Party computers and providing stolen emails to WikiLeaks.

Mr. Brennan, who has not hidden his dislike for Mr. Trump, testified he briefed the investigation’s progress to Mr. Obama, who at the time was trying to aid Hillary Clinton in her campaign against the Republican nominee.

As Mr. Brennan described his actions to the House committee: “I wanted to make sure that every information and bit of intelligence that we had was shared with the bureau [FBI] so that they could take it. It was well beyond my mandate as director of CIA to follow on any of those leads that involved U.S. persons. But I made sure that anything that was involving U.S. persons, including anything involving the individuals involved in the Trump campaign, was shared with the bureau.

“I was aware of intelligence and information about contacts between Russian officials and U.S. persons that raised concerns in my mind about whether or not those individuals were cooperating with the Russians, either in a witting or unwitting fashion, and it served as the basis for the FBI investigation to determine whether such collusion [or] cooperation occurred,” Mr. Brennan added.

Eleven months later, there is no official public confirmation that Trump people colluded with the Russians on hacking.

When Rep. Trey Gowdy of South Carolina, the Republican point man in questioning Mr. Brennan, asked what the Russians and Trump people were talking about, the former top spy said he did not know.

“I saw interaction and [was] aware of interaction that, again, raised questions in my mind about what was the true nature of it. But I don’t know. I don’t have sufficient information to make a determination whether or not such cooperation or complicity or collusion was taking place. But I know that there was a basis to have individuals pull those threads,” Mr. Brennan said.

It is known that some Trump people had contact with Russians during the campaign, when the hacking occurred, and some during the transition.

Jared Kushner, a White House aide and Mr. Trump’s son-in-law, is known to have communicated with Russian Ambassador Sergey Kislyak during the transition, as did retired Army Lt. Gen. Michael Flynn.

The State Department sponsored a trip by diplomats to the Republican National Convention in July. Mr. Kislyak was among those who attended.

One Trump person known to have public Russian contacts in July was Carter Page. Mr. Page signed on as a low-level volunteer who made TV appearances on Mr. Trump’s behalf and offered advice on foreign policy.

Mr. Page, who has done business with Russians for years and lived in Moscow in the 2000s as a Merrill Lynch investment banker, returned last summer to give two talks that were covered by the news media.

Mr. Page has told The Washington Times he played no role in any Russian conspiracy to hack or otherwise interfere in the election.

He believes the Trump campaign severed ties with him because of sensational charges in an unverified anti-Trump dossier that surfaced in a smattering of news stories before Nov. 8.

The dossier was one of the forces influencing the FBI that summer. Some press reports said it was the reason the bureau began investigating Trump associates and acquired a warrant to wiretap Mr. Page as a possible foreign agent.

But Mr. Brennan’s May 23 testimony shows that it was his actions that drove the FBI probe.

The dossier was financed by a Clinton backer and written by British ex-spy Christopher Steele. He was hired by Democratic-tied Fusion GPS in Washington.

Mr. Steele’s 35 pages of memos were first circulated in late June. In mid-July Fusion passed around another memo that made the most sensational charges. “Further Indications of Extensive Conspiracy Between Trump’s Campaign and the Kremlin” was the headline.

Mr. Steele said that Mr. Carter and former Trump campaign manager Paul Manafort formed a team to work with the Russians to hack the Democrats.

Mr. Page calls the charge preposterous. He told The Times he has never met Mr. Manafort.

Also denying the charges was Mr. Manafort, whom the Trump organization fired after reports he received questionable payments from a pro-Russian Ukrainian politician. Mr. Manafort said he did not knowingly talk to any Russians.

After Mr. Brennan’s May 23 appearance, Mr. Page sent a letter to the House committee.

“His testimony followed closely in line with the highly defamatory and baseless accusations offered during their regime’s final year in office as well as the months since,” Mr. Page wrote to Rep. K. Michael Conaway, the Texas Republican who is leading the panel’s investigation, and Rep. Adam B. Schiff of California, the committee’s top Democrat.

“Throughout my interactions with the Russians in 2016, I consistently made it crystal clear that all of my benign statements and harmless actions in Moscow as well as elsewhere overseas were solely made as a scholar and a business person speaking only on behalf of myself. In other words, in no way connected to then-candidate Trump,” Mr. Page wrote.

The Steele dossier said he met with two Kremlin-connected Russians in Moscow that July. Mr. Page said he has never met the two men.

Mr. Brennan has been a harsh critic of Mr. Trump, especially since the election. He took umbrage at Mr. Trump blaming the intelligence community for leaks and his likening it to how the Nazis did business. Mr. Brennan said Mr. Trump does not understand the threat posed by Russia.

While Mr. Brennan was at the White House, the Obama administration launched a six-year “reset” approach to Moscow, with then-Secretary of State Clinton standing next to Russian President Vladimir Putin and urging Americans to do business with Russia.

Relations soured after Mr. Putin’s forces invaded Ukraine.

To this day, nearly a year after Mr. Brennan alerted the FBI, there has been no public official confirmation that Trump people coordinated with the Russians on hacking. Sen. Dianne Feinstein, California Democrat and Senate Intelligence Committee member, said earlier this month she has seen no evidence of collusion.

The Senate and House Intelligence committees are both investigating that charge.

Last week, the Senate panel asked the Trump presidential campaign for all records related to Russia.

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