Osama Bin Laden gave US forces the slip by bribing the Afghan militias tasked with tracking him down, according to Germany's spy chief, August Hanning.
The use of Afghan allies at Tora Bora was a controversial strategy
Mr Hanning told German newspaper Handelsblatt that using Afghans was the key mistake in the hunt for Bin Laden.
He said Bin Laden paid "a lot of money" to buy a safe passage from the Tora Bora caves, which he had retreated to during the US assault in 2001.
The US has said it used Afghan fighters to reduce casualties among its troops.
US commander Tommy Franks has conceded that some of the Afghan groups nominally allied to the US during the invasion may have allowed Taleban and al-Qaeda fugitives to slip away in exchange for money.
He has not said whether Bin Laden was among these fugitives.
Mr Hanning did not directly say US strategy helped Bin Laden get away.
"The principal mistake was made already in 2001, when one wanted Bin Laden to be apprehended by the Afghan militias in Tora Bora," he said.
"There, Bin Laden could buy himself free with a lot of money."
Having fled the assault on Tora Bora, the al-Qaeda lynchpin was able to create a network of sympathisers among tribes in the border region between Afghanistan and Pakistan, Mr Hanning said.
Following the US capture of the Afghan capital, Kabul, in November 2001, the pursuit of Bin Laden focussed on the Tora Bora cave complex, near the Afghan border.
While US jets bombed the area from above, its special forces on the ground worked with Afghan allies to corner surviving Taleban and al-Qaeda fighters.
Some experts at the time argued that the Afghan commanders siding with the US were keener to consolidate their position than to aid the hunt for Bin Laden.