BEIRUT, Lebanon — Scores of men, women and children were killed outside Damascus on Wednesday in an attack marked by the telltale signs of chemical weapons: row after row of corpses without visible injury; hospitals flooded with victims, gasping for breath, trembling and staring ahead languidly; images of a gray cloud bursting over a neighborhood.

But even with videos, witness accounts and testimonies by emergency medics, it was impossible to say for certain how many people had been killed and what exactly had killed them. The rebels blamed the government, the government denied involvement and Russia accused the rebels of staging the attack to implicate President Bashar al-Assad’s government.

Images of death and chaos poured out of Syria after what may be the single deadliest attack in more than two years of civil war. Videos posted online showed dozens of lifeless bodies, men wrapped in burial shrouds and children, some still in diapers. There were hospital scenes of corpses and the stricken sprawled on gurneys and tile floors as medics struggled to resuscitate them.

Getting to the bottom of the assault could well alter the course of the conflict and affect the level of the West’s involvement.