A Rwandan-born man has been arrested in Ohio on charges of hiding his involvement in the 1994 Rwandan Genocide.
Federal prosecutors accuse Eric Tabaro Nshimiye of concealing his involvement in the mass murders, including personally hacking people to death.
Mr Nshimiye has lived in Ohio since 1995 after fraudulently gaining refugee status in the US, prosecutors say.
He has previously denied participating in the genocide.
He is due to appear in federal court in Boston at a later date.
"[Mr] Nshimiye is accused of lying to conceal his participation in one of the greatest human tragedies of all time," Homeland Security Investigations Special Agent Michael Krol said in a statement.
"The government alleges his testimony in the defence of a convicted genocidaire was a calculated attempt to conceal the horrific crimes committed during the genocide."
Mr Nshimiye testified at the 2019 trial of Jean Leonard Teganya, who was convicted as a genocide perpetrator. Prosecutors accuse him of lying under oath in order to conceal his involvement in the killings. He is alleged to have personally participated in killings of ethnic Tutsis by hitting them on the head with a nail-studded club and then hacking them to death with a machete, according to court filings.
He left Rwanda in mid-1994. The following year, he travelled to Kenya, where he is accused of lying to US immigration officials in order to enter the US.
He faces charges of falsifying, concealing and covering up a material fact, obstruction of justice, and perjury.
David Johnson, a defence lawyer for Mr Nshimiye, did not immediately answer a telephone call requesting comment.
The Rwandan Genocide saw about 800,000 people slaughtered in Rwanda by ethnic Hutu extremists over just 100 days in 1994. So-called génocidaires targeted members of the minority Tutsi community, as well as their political opponents, irrespective of their ethnic origin.