Guillermo Novo Sampol voluntarily returned to the federal prison at Danbury, Conn. yesterday, two days after he had been freed mistakenly, and resumed serving a 54-month sentence in connection with the car-bombing assassination of former Chilean ambassador Orlando Letelier.

Novo walked out of the prison Monday after officials confused him with his brother, Ignacio, who was released from custody by a federal judge last week. Both men had been convicted of lying to a federal grand jury investigating the Letelier case.

A spokesman at Danbury said that Guillermo Novo spoke with the prison warden Tuesday night and was advised that his release was an error. Prison officials had double-checked the confusion in names Monday and were assured by the U.S. Marshal's Service in Washington that "Novo Sampol" was to be freed.