Police said the man, believed to be in his 50s, was already unconscious when officers arrived at the Ladbrokes office in Plymouth, The Sun reported. The officers performed CPR but were unable to revive him.
The man, wearing a gas mask and armed with a gun, entered the office at around 6:45 p.m., police said. There were about 12 people in the shop who succeeded in disarming the man and then held him down.
Chief Inspector Ian Drummond-Smith said the death is being investigated.