The extension came amid fresh disclosers of videotapes showing US prison guards punching detainees, tying one to a bed for questioning and forcing a dozen to strip from the waist down.
One squad of guards was all-female, traumatising some Muslim prisoners.
The commander of the US military's Southern Command, General Bantz Craddock, extended the deadline to 28 February and said further extensions could not be ruled out because the investigators needed to reach widely scattered witnesses who had been at Guantanamo.
"These witnesses are located from Maine to California, nationally, and from Iraq to Korea, internationally," Craddock said in a written statement.
The Southern Command, which oversees the detention centre for foreign suspects at the US naval base in Guantanamo Bay, had earlier asked two officers to investigate and report back by 1 February.
The investigation was ordered last month after the public release of FBI documents that described prisoners shackled in foetal positions on the floor for up to 24 hours and left in their own urine and faeces.
"These witnesses are located from Maine to California, nationally, and from Iraq to Korea, internationally"
General Bantz Craddock,
US Southern Command chief
One described an interrogation in which a prisoner was wrapped in an Israeli flag and bombarded with loud music and strobe light. Another reported seeing a barely conscious prisoner who had torn out his hair after being left overnight in a sweltering room.
The American Civil Liberties Union (ACLU) obtained the documents under court order through a freedom of information request and released them.
The Associated Press has meanwhile quoted from a secret report by US military investigators prepared last June, detailing contents of videotapes showing prisoner treatment at Guantanamo.
One video clip showed one or more guards punching a detainee "on an area of his body that seemingly would be inconsistent with striking a pressure point", which is a sanctioned tactic for subduing prisoners.
Another video clip showed a guard kneeing a detainee in the head, while another showed a team of prison guards securing a detainee to a bed for interrogation.
Investigators also noted about a dozen cases where detainees were stripped from the waist down and taken to the "Romeo block" of the camp. It is a section of the detention centre where prisoners were often left naked for days.
Prisoners released from Guantanamo have accused the interrogators of abuse, and one former US National Guardsman suffered brain damage after posing undercover as a rowdy detainee and ending up being assaulted.
Prisoner abuse in US detention
centres might be systematic
"The obvious problem with our armed forces is their inability to comply with international law," Arsalan Iftikhar, national legal director for the Washington, DC-based Council on American-Islamic Relations, said.
"Many of us thought that the Abu Ghraib scandal in Iraq was going to shake us into awakening, but it seems like the things we keep learning about Guantanamo indicate there was, in fact, systematic abuse."
The ACLU has filed a request asking for all photographs and videotapes depicting the treatment of the detainees.
Although a court ordered the government to comply with the ACLU request and turn over documents - thousands of which the ACLU has received - the government has refused to provide videos, citing privacy concerns, Jameel Jaffer, an ACLU attorney, said.