Uganda's Cheptegei was crowned 10,000m world champion in Doha in October, having won the 10km cross country world title in Denmark in March.
The report said the deceased entered the urinals alone but a few minutes later, a cleaner, Milton Mumbere, heard a bang and rushed to check. He found AK-47 on his back.
MALL BRAWL! Black Friday chaos sweeps the world with fights, stampedes and gun scares at shopping centres
Black Friday is an informal name given to a day of highly-promoted sales that began in the US but that now marks the start of the Christmas shopping season in dozens of other countries.