A Nebraska congressman has been found guilty of three felonies for lying to the FBI about illegal campaign contributions from a foreign billionaire.

A federal jury in Los Angeles convicted Jeff Fortenberry, a nine-term Republican, on Thursday of concealing information and two counts of providing false statements to authorities.

He denied to federal authorities that he was aware of illegal campaign donations from Gilbert Chagoury, a Nigerian billionaire of Lebanese descent who disguised his identity through third-party contributions, said the US attorney’s office for the central district of California.

According to court documents reviewed by the Washington Post, Chagoury was accused of making illicit campaign contributions worth up to $180,000 to four American political candidates, including Fortenberry.

One of Chagoury’s associates gave $30,000 to “an individual at a restaurant in Los Angeles, who, along with others, later made campaign contributions” to Fortenberry’s re-election campaign, according to officials.

Chagoury had connections to Defense of Christians, a nonprofit that combated the persecution of Christians and other minorities in the Middle East, court documents revealed. He attempted to funnel money to “politicians from less-populous states because the contribution would be more noticeable to the politician and thereby would promote increased donor access,” said federal prosecutors.

Fortenberry has maintained his innocence. In a YouTube video filmed with his wife and dog, he said “a person from overseas illegally moved money to my campaign” and he “didn’t know anything about this”.

“I feel so personally betrayed … We thought we were trying to help,” Fortenberry said. His attorneys accused federal agents of setting him up.

Fortenberry’s sentencing is set for 28 June. Each count carries up to five years of federal prison time, along with fines. Fortenberry said the court process had been unfair and that he would immediately appeal.

“After learning of illegal contributions to his campaign, the congressman repeatedly chose to conceal the violations of federal law to protect his job, his reputation and his close associates,” said US attorney Tracy L Wilkison in a statement.

“The lies in this case threatened the integrity of the American electoral system and were designed to prevent investigators from learning the true source of campaign funds.”