A masked armed robber collapsed and died after being disarmed and pinned down by customers in a bookies.
The man, in his 50s, was believed to be wearing a gas mask and holding a gun when he stormed into the Ladbrokes branch on Crownhill Road in Plymouth, Devon, shortly before 7pm last night.
Brave customers disarmed the man, Devon and Cornwall Police said, and held him on the floor while others raised the alarm.
Police arrested the man, who was already unconscious, upon arrival, but he was declared dead a short time later.
The police watchdog, the Independent Police Complaints Commission (IPCC), has been informed of the incident.
A police spokesman said: "It quickly became clear the man was unresponsive and an ambulance was immediately requested by police who carried out CPR until they arrived.
"A few minutes later three paramedics arrived at the scene and continued to carry out further CPR. Around 20 minutes later the man was declared dead at the scene by the paramedics.
"This incident has been voluntarily referred to the Independent Police Complaints Commission."
Chief Inspector Ian Drummond-Smith said: "This is a very serious incident. We cannot speculate on what has actually taken place here today.
"A full investigation has now begun and while the IPCC carry out their investigation it is not appropriate to comment further."
The man has been identified but next of kin have not yet been informed, police said.
Ladbrokes declined to comment when contacted last night.