KABUL, Afghanistan — Their last meal was a picnic in the forest in the Sharrun Valley, high in the Hindu Kush mountains of northern Afghanistan.

Returning home from a three-week trek on foot to deliver free medical care to the remotest regions of the country, the aid workers — six Americans, a Briton, a German and four Afghans — had just finished eating when they were accosted by gunmen with long dyed-red beards, the police said.

The gunmen marched them into the forest, stood them in a line and shot 10 of them one by one.

The police found their bodies, seven men and three women, on Friday, the Badakhshan Province police chief, Gen. Aqa Noor Kentoz, said Saturday.

The attack, the largest massacre in years of aid workers in Afghanistan, offered chilling evidence of the increasing insecurity in the northern part of the country and added to fears that the insurgency has turned even more vicious in recent months.