Meeting with reporters in Germany, Holder also signaled the Obama administration might cooperate with a Spanish investigation of former Bush administration officials over the treatment of terror suspects.
Holder spoke hours ahead of a speech appealing for Europe's help in closing the military detention facility at the U.S. naval base in Cuba. He said the U.S. is weeks away from asking certain countries to take detainees.
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"We have about 30 or so where we've made the determination that they can be released. So we will, I think, relatively soon, be reaching out to specific countries with specific detainees and ask whether or not there might be a basis for the moving of those people from Guantanamo to those countries," Holder said.
There are 241 prisoners at Guantanamo, and Holder has been visiting European leaders this week asking for help relocating detainees as he seeks to carry out President Barack Obama's order to shutter the detention site over the next nine months.
Previewing his later speech, Holder said that while the U.S. created the Guantanamo site, closing it is a shared responsibility for the U.S. and its allies.
"Mistakes were made" in the creation of the post-Sept. 11 Guantanamo program, Holder said, but he has been telling European officials over the past week that "the problem that it created is best solved by a unified response."
Closing Guantanamo is good for all nations, he argued, because anger over the prison has become a powerful global recruiting tool for terrorists.
Yet when it comes to the prospect of having former international terror suspects living free, the Obama administration is trying to overcome the not-in-my-backyard sentiment that exists on both sides of the Atlantic.
Holder met with German officials Wednesday to discuss Guantanamo and other legal issues, following similar meetings in London and Prague.
Several European nations, including Portugal and Lithuania, have said they will consider taking such detainees. Others, like Germany, are divided on the issue.
French President Nicolas Sarkozy already has made what was billed as a symbolic gesture of agreeing to take one Guantanamo detainee.
In his comments Wednesday, Holder also said it is possible the U.S. could cooperate with a foreign court's investigation of Bush administration officials.
Speaking before it had been announced that a Spanish magistrate had opened an investigation of Bush officials on harsh interrogation methods, Holder didn't rule out cooperating in such a probe.
"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.
Pressed on whether that meant the U.S. would cooperate with a foreign court prosecuting Bush administration officials, Holder said he was talking about evidentiary requests, and would review any such request to see if the U.S. would comply.