"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.
No decisions have yet been made to designate any detainee for indefinite detention, officials said.
Some senators questioned how the administration planned to prosecute detainees in federal court when most were never given Miranda warnings at the time they were captured. Miranda warnings are an American legal rule requiring a defendant be notified of their right to remain silent before being questioned.
Administration critics suggest the plan will inevitably force US soldiers on foreign battlefields to read battlefield prisoners their rights when they should be engaged in combat operations and intelligence gathering.
Kris said the administration would not require soldiers to issue Miranda warnings. He said in a few cases, FBI agents had given such warnings before questioning suspects. "You need to approach these national security problems one at a time. It will vary from case to case," he said. Kris said if Miranda warnings are given in a case it would keep the criminal prosecution option open in that case. If no warnings were given, he said, the administration would have the flexibility to turn that suspect over to a military commission with more relaxed rules about admission of involuntary statements.
Sen. Jon Kyl (R) of Arizona questioned how any detainees would eventually qualify for a civilian court trial since the vast majority of Guantánamo detainees never received Miranda warnings at the time they were captured.
Kris said in some cases there may be an independent source of evidence against a detainee-suspect. He said flexibility would be the key. "I think the concern you are getting at, you don't want the tail to end up wagging the dog," Kris said. "That is a legitimate concern, but we have enough flexibility under the protocol to protect against that."