The US government opposed releasing Luis Posada Carriles, an anti-Castro militant wanted by Venezuela in the 1976 bombing of a Cuban airliner that killed 73 people, court papers show.
A judge last month recommended that Posada Carriles, who is being held on immigration charges, be freed because no country was found to accept him, other than Cuba and Venezuela, both of which want to try him for a series of bombings.
The US Justice Department said in its filing on Thursday that Posada Carriles, 78, should look harder for a country that will receive him, given his "uncommon ability to facilitate his own removal through high-level government connections in multiple Latin American countries."
"Posada identified an extraordinary range of foreign high profile contacts and prospects for removal that he only partially pursued," it said.
The court filing comes on the eve of the Cuban airline bombing's 30th anniversary.
Venezuela, of which Posada Carriles is a citizen, said the United States should extradite him to stand trial in the bombing.
But it has also failed to find another third country that would accept him.
The United States refused, saying he might then be handed over to Cuba, which also says he committed terrorist bombings and killed an Italian citizen in the process.
Posada Carriles has been in a Texas jail since May 2005 for entering the United States illegally.