A federal jury in Los Angeles found the nine-term lawmaker guilty of one count of scheming to falsify and conceal material facts and two counts of making false statements to federal investigators. Each count carries a maximum penalty of five years in prison, as well as fines.
He told reporters outside the courthouse that he would appeal.
Fortenberry, a member of the House Appropriations Committee, was charged in October with lying to the FBI about a $30,000 contribution to his 2016 re-election campaign from Gilbert Chagoury of Nigeria. Foreign nationals are prohibited from contributing to federal candidates in U.S. elections.
Prosecutors said Chagoury used "straw donors" to make contributions equaling $30,000 to Fortenberry's re-election campaign during a Los Angeles fundraiser in 2016.
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Chagoury entered into a deferred prosecution agreement with the U.S. Attorney's Office in 2019 and came clean about providing roughly $180,000 that was used to make illegal contributions to four political candidates in U.S. elections.
During Fortenberry's 7-day trial, prosecutors successfully argued that the lawmaker lied to investigators on two occasions when asked what he knew about the illegal donation during interviews.
Fortenberry, 61, also failed to file an amended report with the Federal Election Commission.
"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," U.S. Attorney Tracy L. Wilkison said in a statement Thursday. "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."
The jury deliberated for about two hours before returning the guilty verdicts.
Fortenberry falsely told investigators he was not aware that one of Chagoury's ties — Toufic Joseph Baaklini — was involved in illegal campaign contributions when the lawmaker spoke with investigators in March 2019, after learning of the illicit contribution, prosecutors said. They also said Fortenberry claimed that donors at the 2016 fundraiser were all publicly disclosed and that he was not aware of any contributions from a foreign national.
During a second interview two months later, Fortenberry denied awareness of any illicit donation made during the 2016 fundraiser, or that he had been told Baaklini provided $30,000 in cash at the fundraiser, prosecutors said.
A judge set Fortenberry's sentencing for June 28.
It is unclear whether whether Fortenberry will stay in Congress, or what repercussions he could face if he does not step down.
NBC News has reached out to Fortenberry's office for comment.
Fortenberry is on the ballot for the GOP primary in May and faces five challengers, including state Sen. Mike Flood, who is seen as the strongest contender. Flood has cited Fortenberry's legal woes in his congressional bid.
Fortenberry was first elected to Nebraska's 1st Congressional District in 2004 and has often cruised to re-election in the reliably red state. In 2020, he defeated his Democratic opponent by winning 59 percent of the vote.