WASHINGTON — The Justice Department on Tuesday charged a Republican member of Congress from Nebraska with lying to the F.B.I. during a campaign finance investigation, an allegation that the lawmaker vowed to fight.

The Justice Department accused the lawmaker, Representative Jeff Fortenberry, of lying to the F.B.I. twice about whether he knew that he had received illegal campaign donations, including during an interview with the government that his lawyer attended, according to the federal indictment.

Anticipating that the department intended to charge him, Mr. Fortenberry said in a video posted online on Tuesday morning that F.B.I. agents unexpectedly came to his home two years ago to question him about the possibility that he had received illegal campaign donations.

“I told them what I knew and what I understood,” Mr. Fortenberry said. “They’ve accused me of lying to them and are charging me with this.” He called the possibility of criminal charges shocking and stunning.

The indictment stems from a separate federal investigation into Gilbert Chagoury, a Lebanese Nigerian billionaire who was accused of conspiring to make illegal campaign contributions to American politicians in exchange for access to them.

Foreign citizens are prohibited by federal law from contributing to U.S. election campaigns. Mr. Chagoury admitted this year to providing approximately $180,000 to four candidates from June 2012 to March 2016. He said he had used others, including Toufic Joseph Baaklini, a Washington lobbyist, to mask his donations.

Mr. Fortenberry, who has served in Congress for 15 years, was one of those politicians. He is not disputing the fact that the donations, ultimately from Mr. Chagoury, were illegal.

“Five and a half years ago, a person from overseas illegally moved money to my campaign,” Mr. Fortenberry said in his video. “I didn’t know anything about this.”

Mr. Fortenberry is not being accused of helping Mr. Chagoury in his scheme. Rather, prosecutors are looking at whether the congressman lied when they asked him in 2019 whether he was aware that some contributions were illegally made to his campaign in 2016.

The government said in court filings that in spring 2018, one of Mr. Fortenberry’s fund-raisers told the congressman that he had funneled $30,000 from Mr. Baaklini to the 2016 re-election event, but that the money “probably did come from Gilbert Chagoury.”

The fund-raiser, referred to as Individual H in the indictment, was cooperating with law enforcement when he spoke with Mr. Fortenberry, according to the indictment.

Despite the fact that the donations were most likely illegal, Mr. Fortenberry did not take appropriate action, such as filing an amended report with the Federal Election Commission or returning the contributions, the indictment said. It was not until after the Justice Department contacted him in July 2019 that Mr. Fortenberrry returned the contributions, according to the document.

In his initial interview with the F.B.I. in 2019, Mr. Fortenberry said that the people who had contributed during his fund-raising event in 2016 were all publicly disclosed, and that he was unaware of any contributions made by foreign citizens, according to the indictment.

During a subsequent interview at the office of Mr. Fortenberry’s lawyer, the Justice Department alleged that Mr. Fortenberry “falsely stated that he had not been told by Individual H during the 2018 call that Baaklini had given Individual H $30,000 cash” to funnel into his campaign, and that “he was not aware of any illicit donation made” during the fund-raising event.

Mr. Fortenberry told investigators that he had ended the 2018 call with the government’s cooperating witness after that person had made a “concerning comment,” even though the indictment alleged that the witness went on to “repeatedly and explicitly” describe illegal contributions and referred to an illegal contribution from a foreign national.

“We will fight these charges,” Mr. Fortenberry said in his video. “I told them what I knew.”

He has known about the possible charges for at least the past few weeks; he used the existence of the investigation in an effort this month to raise money for a legal-defense fund. That campaign was first reported by Axios.

Prosecutors said in court documents that Mr. Chagoury was advised to donate to “politicians from less-populous states because the contribution would be more noticeable to the politician and thereby would promote increased donor access.”

Mr. Chagoury entered into a deferred prosecution agreement with the Justice Department in 2019. Under that agreement, he admitted to wrongdoing. The department can use those admissions in other matters. He also agreed to cooperate with prosecutors in their investigation. In return, the U.S. government agreed to drop the charges. The matter was ultimately resolved this year, when Mr. Chagoury paid a $1.8 million fine.