U.S. officials said late Tuesday that they believe the Russian military dropped the bombs on a United Nations aid convoy in Syria that killed more than 20 people on Monday during a tenuous cease-fire. More than a dozen vehicles in the aid convoy were hit. Workers were unloading medicine and food at a warehouse when one bomb fell from the sky in rural Aleppo. Two Russian warplanes were above the convoy when it was hit, unnamed U.S. officials have said. Outgoing Secretary General Ban Ki-moon described the attack as “sickening, savage, and apparently deliberate.” Other UN officials called the airstrike a war crime. Ban said, “Those who bombed them were cowards.” He added, “Just when you think it cannot get any worse, the bar of depravity sinks lower.” Meanwhile, four aid workers were reported killed overnight Tuesday when an airstrike hit a medical center outside Aleppo in the rebel-held town of Khan Touman. Two nurses and two ambulance drivers with the Union of Medical Care and Relief Organizations charity are reported to have died as they raced to help victims of another attack.