"I felt Guantanamo should be closed for the past six years, and I lobbied and presented reasons to President (George W.) Bush," Powell said in an interview with CBS television.
With former vice president Dick Cheney leading the charge, Republicans have attacked Obama for promising to close the prison camp by January without a plan for dealing with the estimated 240 terror suspects held there.
Democratic lawmakers also have opposed transferring potentially dangerous prisoners to US prisons, forcing Obama to defend his "war on terror" policies in a major speech this week.
In an interview with ABC television, Mullen said he had long been an advocate for closing the prison because it "has been a recruiting symbol for those extremists and jihadists who would fight us."
Asked about Cheney's charge that the recruitment argument amounted to blaming "America for the evil others do," Mullen said, "It's my judgment that (Guantanamo) has had an impact (on recruiting). And it's time to move on."
He acknowledged the difficulties of figuring out what to do with suspects who are too dangerous to release but cannot be tried, and how to ensure that those released do not return to the fight.
He said detainees released from Guantanamo have returned to the battlefield in increasing numbers over the last year or two, and in recent months the percentage of recidivism has climbed from "five or six percent to the low teens."
"We're working hard now to figure out what the options are and what the best one would be," he said. "And that really is a decision the president is going to have to make, certainly in meeting this deadline of what we do."
Powell also took issue with Cheney's criticism, saying that even Bush had turned against Guantanamo by the end of his administration.
"President Bush stated repeatedly to international audiences and to the country that he wanted to close Guantanamo. The problem he had was he couldn't get all the pieces together," Powell said.
"So it is a complex problem, and president Bush wasn't able to close Guantanamo on his watch. And President Obama came in saying he would close Guantanamo, and he has run into some of those same sorts of problems," he said.
"So I think we need to kind of take the heat out of this issue," Powell said.
But members of Congress took aim at the notion of transferring Guantanamo detainees even to super-secure prisons that have housed other terrorist suspects without incident.
Senator Ben Nelson, a Democrat who voted to deny the administration funds for closing Guantanamo, said detainees accused of violating "the laws of war" should not be brought to the United States.
"I think they need to be kept elsewhere, wherever that is," he told Fox News. "I don't want to see them come on American soil."
Senator Barbara Boxer, another Democrat, told CNN: "We want to close it down, but we want to wait and see what the plan is."
She added that her state of California had one maximum security prison -- "and it's right now overbooked."
Senator Jon Kyle, a Republican from Arizona, said he did not think he could be convinced to support the transfer of prisoners from Guantanamo to the United States.
"I don't know why it is better to have somebody in a so-called supermax facility in, say, Colorado than it is to keep them in Guantanamo, a state-of-the-art facility that we built not too long ago for the explicit purpose of holding these people," he told Fox News.
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