The June 22 attack at the base camp for the 8,126-metre Nanga Parbat, Pakistan's second-highest mountain -- nicknamed for its treacherous terrain -- was the deadliest assault on foreigners in the troubled nuclear-armed country for a decade.
Through interviews with multiple officials, militants and negotiators assigned to bring the culprits out of hiding, AFP has been able to piece together a picture of the events surrounding the slaughter and its aftermath.
One year on, with tourism in the northern Gilgit-Baltistan region still suffering, most of the 10 suspects implicated in the attack are still at large, while sources close to the investigation have cast doubt on the guilt of some of those arrested.
But dual national Chen was the prime target, according to militant sources.
Also Read
The story begins in early June 2012, when a local jihadist contacted other fighters to tell them two mysterious commanders had arrived from out of town and wanted to meet.
Pakistan's northern Gilgit-Baltistan region, high in the Himalayas, has been relatively immune to the Islamist insurgency plaguing the country in recent years.
But the remote area has served for decades as a nursery for fighters to wage war in neighbouring Indian-administered Kashmir, as well as sectarian violence.
The local fighters were briefed on the planned Nanga Parbat operation.
"They were told that the mission was about kidnapping a foreigner in order to later bargain for the release of an important Taliban commander," an investigator assigned to the case said.
Militant sources said that the Chinese-American was the specific target, with the plan being to trade him for Taliban in Afghanistan.
The men then met a local sectarian group in a forest, recruiting two more fighters -- against the sectarian group leader's wishes -- to bring their number to 10.
But as the attack unfolded in the freezing night, Chen burst out of his tent and tackled one of the militants using martial arts techniques, military sources said.