The New York Times spoke to 20 local and international aid workers, rescuers and residents who either witnessed the attack or were involved in preparing the convoy, and reviewed dozens of videos, photographs, social media posts and records kept by aircraft spotters. Together, the interviews and other material indicate that there was a sustained, coordinated attack carried out by Russian or Syrian aircraft, probably both.

About 30 explosions erupted, starting after 7 p.m. and lasting for hours, the interviews and documentation show. The blasts created large fireballs over the warehouse, and set trucks aflame amid the sound of helicopters and jets in the sky. A second — and, some said, a third — wave hit as rescuers tried to pull out the dead and injured, driving them back.

The convoy had come from government territory, with meticulously extracted permissions, and was marked conspicuously with the logos of the United Nations and the Red Crescent. But it did not, as many such convoys do, have the extra shield of United Nations staff members on board, because the Syrian government had blocked them.

As always, on board were volunteers from the Syrian Arab Red Crescent, which is state-supervised in government areas but also has self-governed local branches in rebel territory. Dozens of its volunteers have been killed or jailed during the war.

Mr. Barakat, a father of six, was a crucial link in divided and devastated Aleppo Province. His ties to rebel groups and clerics in rebel-held areas as well as to his aid-worker colleagues from the government-held section of Aleppo city had previously made it safe for international Red Cross officials to visit the organization’s warehouse in Urum al-Kubra.

This time, though, would prove different.

A convoy was approved, but not United Nations attendants

The cease-fire was a week old and fraying when the green light came. The Syrian Foreign Ministry, which must approve all aid deliveries from government territory, authorized the convoy to depart on the morning of Sept. 19, said Kevin Kennedy, the United Nations’ regional humanitarian coordinator in Amman, Jordan.

Many pairs of eyes — from the Syrian government, the United Nations and the Red Crescent — watched as the trucks were loaded at Red Crescent headquarters in the government-held section of Aleppo. Government security officials searched the trucks and saw them sealed, to make sure only approved items were inside: flour, food, blankets and winter clothes for 78,000 people.