Identified in a federal court hearing Tuesday only as John Doe, the man is believed to be Kefelegn Alemu Worku, a high-ranking guard at the "Higher 15" prison in the late 1970s, The Denver Post reported.
Doe was spotted in May at an Aurora, Colo., restaurant by a man identified in court as K.K. who had been a prisoner at the facility.
The suspect was using the name Habteab Temanu when he was seen in the restaurant.
K.K. had not been tortured, but had watched Worku torture prisoners, Homeland Security Investigations special agent Jeffrey Lembke testified.
Two other Ethiopians held at the prison but who now live in Virginia also identified as Worku the photo of the man who identified himself as Temanu.
A former guard at the prison named Kefelegn Alemu was convicted in absentia of taking part in the execution of 101 people, the BBC reported in 2001. Alemu was ordered hanged if found.
Federal authorities have charged the man alleged to be Worku with identity theft and immigration offenses, saying he lied on immigration documents.
He is not charged in connection with the Ethiopian crimes.
Matthew Golla, the man's attorney, asked Magistrate Judge Kristen Mix to release his client on bond, saying he had no criminal history in the United States.
Prosecutor Brenda Taylor argued that federal records suggest the suspect had used at least three different aliases in the United States, raising the possibility that he might flee.
Mix ordered the man held without bond.