The revelation was made in an report on new Af-Pak policy entitled 'Tora Bora Revisited: How we failed to get Bin Laden and Why it Matters Today".
"On or around December 16, two days after writing his will, bin Laden and an entourage of bodyguards walked unmolested out of Tora Bora and disappeared into Pakistan's unregulated tribal area. Most analysts say he is still there today," the report said.
The report contains severe criticism on then defence secretary Donald Rumsfeld and his top military commander Tommy Franks for their decision to relay on air strikes instead of allowing reinforcements to go after Osama and allowing him to escape.
"The vast array of US military power, from sniper teams to the most mobile divisions of Marine Corps and the Army, was kept on the sidelines, instead, the US command chose to rely on airstrikes and untrained Afghan militias to attack bin Laden and on Pakistan's loosely organised Frontier Corps to seal his escape routes," the report said.
Blaming Rumsfeld and Franks for causing US its most fatal mistake, the report said that the lost opportunity spiralled into the growth of insurgents in Pakistan and Afghanistan.
"Our inability to finish the job in late 2001 has contributed to a conflict today that endangers not just our troops and those of our allies, but stability of a volatile and vital region," John Kerry, the chairman of the Senate foreign relations committee, wrote in the report.
The report was presented at the backdrop of president Barack Obama's decision to deploy more troops in Afghanistan.