BARLOVENTO, Venezuela—The young men had already been tortured at an army base when soldiers piled them into two jeeps and transported them to a wooded area just outside the Venezuelan capital.

Stumbling in the dark, with T-shirts pulled over their faces and hands tied behind their backs, they were steered to an open pit. Soldiers then used machetes to deliver blow after blow to the base of their necks. Most suffered gaping wounds that killed them before they hit the ground.

Others,...