KABUL, Afghanistan — After Shir Mohammad Panjshiri shoveled a final spadeful of dirt onto his brother’s grave, he left the cemetery burdened with questions.

His brother, Aziz Ahmad Panjshiri, a retired college professor, had been abducted from a car in northern Afghanistan on Nov. 5. He was later shot dead, his body dumped in a neighboring province.

His grieving brother still did not know why.

“What happened to my brother isn’t clear,” Mr. Panjshiri said after Professor Panjshiri, 69, was buried on a Kabul hillside. “We’re waiting for the government to tell us.”

Afghan roads are some of the most dangerous in the world, turned into killing grounds through years of bitter guerrilla warfare. Off-duty police officers and soldiers, judges, government officials, human rights workers, businessmen, prosecutors — all have been killed on highways in Afghanistan’s rugged countryside.