Osama bin Laden was unquestionably within reach of US troops in the mountains of Tora Bora when American military leaders made the crucial and costly decision not to pursue the terrorist leader with massive force, a Senate report says.

The report asserts that the failure to kill or capture bin Laden at his most vulnerable in December 2001 has had lasting consequences beyond the fate of one man. Bin Laden’s escape laid the foundation for today’s reinvigorated Afghan insurgency and inflamed the internal strife now endangering Pakistan, it says.

Staff members for the Senate Foreign Relations Committee’s Democratic majority prepared the report at the request of the chairman, Sen. John Kerry, as President Barack Obama prepares to boost US troops in Afghanistan.

The Massachusetts senator and 2004 Democratic presidential candidate has long argued the Bush administration missed a chance to get the al-Qaida leader and top deputies when they were holed up in the forbidding mountainous area of eastern Afghanistan only three months after the September 11, 2001, terrorist attacks.

Although limited to a review of military operations eight years old, the report could also be read as a cautionary note for those resisting an increased troop presence there now.

More pointedly, it seeks to affix a measure of blame for the state of the war today on military leaders under former president George W. Bush, specifically Donald H. Rumsfeld as defense secretary and his top military commander, Tommy Franks.

“Removing the al-Qaida leader from the battlefield eight years ago would not have eliminated the worldwide extremist threat,” the report says. “But the decisions that opened the door for his escape to Pakistan allowed bin Laden to emerge as a potent symbolic figure who continues to attract a steady flow of money and inspire fanatics worldwide. The failure to finish the job represents a lost opportunity that forever altered the course of the conflict in Afghanistan and the future of international terrorism.”

The report states categorically that bin Laden was hiding in Tora Bora when the US had the means to mount a rapid assault with several thousand troops at least. It says that a review of existing literature, unclassified government records and interviews with central participants “removes any lingering doubts and makes it clear that Osama bin Laden was within our grasp at Tora Bora.”

On or about Dec. 16, 2001, bin Laden and bodyguards “walked unmolested out of Tora Bora and disappeared into Pakistan’s unregulated tribal area,” where he is still believed to be based, the report says.

Instead of a massive attack, fewer than 100 US commandos, working with Afghan militias, tried to capitalize on air strikes and track down their prey.

“The vast array of American military power, from sniper teams to the most mobile divisions of the Marine Corps and the Army, was kept on the sidelines,” the report said.