BAYDA, Syria—The chain of events that led to one of the Syrian war's worst mass killings started when government forces came to this village and arrested Hassan Othman on charges of instigating rebellion.

Mr. Othman, a 31-year-old with a wife and two children, was severely tortured, according to a cousin's account. He confessed to Syrian authorities that rebels secretly operating in Bayda had hidden arms there, say residents, local security officials and opposition activists.

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