The lengths to which the FBI relied on the Hillary Clinton opposition research dossier to investigate her political rival came into sharper focus with the testimony of the lead investigator, FBI agent Peter Strzok.

Mr. Strzok told a joint House committee last week that the FBI received from Associate Attorney General Bruce Ohr a raft of documents — including sections of the anti-Trump dossier — during the investigation into Russia election interference.

The first-time disclosure is significant because it confirms an unusual and continuing channel for collusion accusations that started outside the government with anti-Trump people and reached Mr. Ohr at the Justice Department and then the FBI.

Around that time, Mr. Ohr was communicating with dossier writer Christopher Steele. Mr. Ohr’s wife, Nellie, a Russia researcher, worked for Mr. Steele’s paymaster, investigative firm Fusion GPS, which was trying to damage Republican presidential candidate Donald Trump. Mr. Ohr met with Fusion co-founder Glenn Simpson after the November 2016 election.

Mr. Strzok, who led the FBI investigation of the Trump campaign, testified that he began receiving dossier sections in September 2016, but not from Mr. Ohr, and continued receiving documents into 2017. Mr. Strzok testified that he met with Mr. Ohr during that time.

The arrangement suggests that the FBI used the partisan, anti-Trump dossier and was eager to receive its episodic chapters from June to December 2016.

None of the Russia-Trump collusion charges has been publicly confirmed to date. Mr. Steele’s sources resided in the Kremlin.

Mr. Steele, a former British spy who told Mr. Ohr he wanted to destroy the Trump candidacy, ultimately was paid by the Clinton campaign and the Democratic National Committee.

Mr. Strzok’s text messages at the time revealed that he despised Mr. Trump and wanted, in his word, to “stop” him. He told the House Judiciary Committee and Oversight and Government Reform Committee task force that his FBI legal handlers placed tight controls on what he could disclose about his handling of the dossier.

The agent testified that he couldn’t specify the “various places” where the FBI received dossier information or when he learned that Mrs. Clinton had funded it.

But he did confirm: “I have been directed that I may state that I have read the dossier, that I read the dossier as it came in in parts and pieces to the FBI.

“Mr. Ohr gave the FBI documents, which included material that I believe originated from Mr. Steele,” Mr. Strzok testified.

Rep. Jim Jordan, Ohio Republican, had the job of trying to pry open dossier secrets from Mr. Strzok, a career counterintelligence agent.

Mr. Jordan read from a Strzok email to colleagues. The email said Mr. Simpson and David Corn, a Mother Jones magazine writer who formed a close bond with Mr. Steele, also provided the FBI with dossier claims.

Mr. Steele accused Mr. Trump of supporting an “extensive conspiracy” with the Kremlin to interfere in the presidential election. He made specific charges of collusion against Trump people that remain unconfirmed publicly two years later.

At the end of Mr. Strzok’s testimony, Republicans gained a clearer picture of how the FBI acquired the Democrat-financed dossier that came to dominate the bureau’s investigation yet contained so many shaky charges.

Republicans now know that the dossier trail started with Mr. Steele, Mr. Ohr, Fusion GPS and Mr. Simpson and a journalist. Mr. Corn has denied he worked with the FBI.

Republicans knew beforehand that Sen. John McCain, Arizona Republican, hand-delivered a copy of the dossier to FBI Director James B. Comey in December 2016. BuzzFeed posted that copy on Jan. 17, 2017.

“This is the first time to my knowledge that the FBI has admitted they got parts of the dossier from Bruce Ohr, a fellow DOJ employee. That’s scary,” Mr. Jordan said.

“Bruce Ohr, the fourth-ranking official at the Department of Justice, his wife works for Fusion GPS in the summer. He gets information and passed it to the FBI. That becomes the basis to spy on the Trump campaign, plain and simple,” Mr. Jordan said. “This is the first time to my knowledge the FBI has admitted that, and it was good that we got some of that information out today.”

According to separate House and Senate investigations, the FBI fired Mr. Steele around election time for violating its rules and going to Mother Jones with his story of a wide Trump-Russian conspiracy.

Yet his research continued to reach the FBI via Mr. Ohr. The Justice Department demoted Mr. Ohr after these disclosures.

Before the joint hearing, Republicans unearthed other findings on the FBI-dossier marriage.

For example, the bureau cited Mr. Steele’s work to persuade a federal judge to approve wiretaps on Trump volunteer Carter Page from October 2016 to the following fall.

Mr. Page, a pro-Russian energy investor, visited Moscow in July 2016 to deliver a public commencement address. Mr. Steele subsequently wrote that he met with two Kremlin figures sanctioned by the U.S. government. Mr. Steele said Mr. Page discussed bribes for sanctions relief.

Mr. Page denied the charges under oath repeatedly and said he never met the two Kremlin figures. He has not been charged.

Mr. Steele also wrote that Mr. Page and Paul Manafort, Mr. Trump’s former campaign manager, conspired with the Kremlin to interfere in the election. Mr. Page testified that he had never met nor spoken with Mr. Manafort.

Committees also previously discovered that the FBI agreed to hire Mr. Steele, with an initial $50,000 payment, to continue investigating Mr. Trump. It would have meant the FBI was counting on a Democratic Party hired gun to investigate Mr. Trump during the transition and presidency.

Justice Department Inspector General Michael E. Horowitz is investigating how the FBI decided to open its probe, seek surveillance warrants and hire at least one confidential human source to spy on some Trump people.