Wilkerson said he discussed the issue with Powell, who left his job in 2005.
"I learnt that it was his view that it was not just vice president Cheney and secretary Rumsfeld, but also president Bush who was involved in all of the Guantanamo decision-making," Wilkerson wrote.
"Many of the prisoners detained at Guantanamo had been taken into custody without regard to whether they were truly enemy combatants, or in fact whether many of them were enemies at all," Wilkerson wrote.
By late August 2002 "I found that of the initial 742 detainees that had arrived at Guantanamo, the majority of them had never seen a US soldier in the process of their initial detention and their captivity had not been subjected to any meaningful review."
Children "as young as 12 and 13 and elderly as old as 92 or 93 had been shipped to Guantanamo," Wilkerson said, sold to US forces for 5,000 dollars a head.
Often there was "absolutely no evidence relating to the detainee that was turned over, so there was no real method of knowing why the prisoner had been detained in the first place."
Cheney and Rumsfeld did not want to release innocents because they feared it would reveal the "incredibly confused" detention operation, Wilkerson said.
"Their view was that innocent people languishing in Guantanamo for years was justified by the broader war on terror and the capture of the small number of terrorists who were responsible for the September 11 attacks, or other acts of terrorism," Wilkerson said.
Their detention "was deemed acceptable if it led to a more complete and satisfactory intelligence picture with regard to Iraq, thus justifying the administration's plans for war with that country," he added.
Wilkerson also alleged that Cheney "had absolutely no concern that the vast majority of Guantanamo detainees were innocent... If hundreds of innocent individuals had to suffer in order to detain a handful of hardcore terrorists, so be it."
Wilkerson's statement was filed in support of Adel Hassan Hamad, a Sudanese man held at Guantanamo Bay from March 2003 until December 2007. He claims he was tortured by US agents and filed a damages action Thursday.
President Barack Obama, who followed Bush in the White House, has pledged to close the controversial detention facility.
His administration faces stern domestic opposition to releasing the prisoners on US soil, and has prodded allies, often in vain, to take in prisoners cleared of charges who cannot be returned to their home countries.
Some 183 detainees remain at the US military prison at Guantanamo Bay on Cuba, including dozens already cleared for release. Most have been held without charge or trial.
Wilkerson has been an outspoken critic of the Bush administration since he left the US State Department in 2005.