Garzon is opening a separate, broader probe that does not name any specific suspects but targets "possible material authors" of torture, accomplices and those who gave torture orders.
Garzon is acting on his own, rather than in response to a complaint filed with the National Court, which is the usual procedure for universal justice probes in Spain.
U.S. Attorney General Eric Holder, speaking with reporters in Berlin before the investigation was announced, did not rule out cooperating with such an investigation.
"Obviously, we would look at any request that would come from a court in any country and see how and whether we should comply with it," Holder said.
"This is an administration that is determined to conduct itself by the rule of law and to the extent that we receive lawful requests from an appropriately-created court, we would obviously respond to it," he said.