The administration also tells Congress Tuesday that it will decide the status of all 229 detainees by Oct. 1.
"We remain committed to that deadline," Defense Department General Counsel Jeh Johnson told members of a Senate subcommittee.
Administration officials also testified that they would complete a review of the cases of each of the 229 detainees at Guantánamo by Oct. 1. Roughly half have already been reviewed, with more than 50 approved for transfer to a different country.
The reviews are necessary to decide which of the remaining detainees will be subject to prosecution, either in a reformed military commission process or in civilian federal courts in the US.
"There is going to be a very aggressive working up of these cases," Assistant Attorney General David Kris told the panel. "We want swift and sure justice and we want to get it right."
Mr. Kris, who head the Justice Department's national security division, said a "significant number" of detainees have been approved for prosecution. He declined to offer a precise number.
The administration is attempting to stake out a middle ground between those who favor closing Guantánamo and ending any reliance on military commissions, and those who want Guantánamo and the military commission process to continue to function.
The administration has announced that it will approach future Guantánamo detainee cases with a presumption that they will be prosecuted in civilian federal courts in the US. If that option is not a good fit, officials say, they will consider the option of a military commission trial or indefinite detention without charge.