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Startled residents were disturbed from their morning cornflakes as an air-raid siren rang out.
Many living in Loose, near Maidstone, said they heard the loud noise continuously for five minutes, at around 9.10am.
Former Kent Police Ch Insp Dave Berry, who used the alarms during training, said the sound was unmistakable.
"It was a surreal experience," he said. "I was out in the garden and there was a full air-raid siren.
"Thirty years ago when we used to do practices for nuclear war it made your blood run cold.
"The kind of thing they can be used for is a chemical leak, those sorts of things - it made me feel slightly on edge."
The 51-year-old explained how the sound brought back chilling memories of growing up with the constant threat of nuclear war with Russia.
"You read all the things about Putin and you never know," he said.
"Growing up in the Cold War, when we were children, nuclear war was a very tangible thing. It was most unsettling."
However, the mystery source of the air-raid siren has now been identified.
"You read all the things about Putin and you never know..." - Dave Berry
At around 9.40am police confirmed the alarm was used by Sutton Valence School for an evacuation drill - putting the minds of residents at ease.
A school spokesman said: "In accordance with guidelines from the Health and Safety Executive and the Police, all schools are required to have an evacuation procedure in place to be deployed in the event of an intruder on site.
"This involves evacuating all pupils and staff from a school site.
"To signal the start of the exercise, an evacuation alarm was sounded at the school in North Street, Sutton Valence. This alarm sounds like a Second World War air raid siren and is particularly loud.
"The volume is essential to alert all areas of the school’s 100-acre site with a single alarm. It is
important that the evacuation alarm sounds distinctly different to the fire alarm in order to alert everyone to completely evacuate the school site, rather than simply gather at muster stations.
"Parents of pupils, local residents, services and business were forewarned about the exercise and associated evacuation alarm, but the school would like to apologise for any inconvenience caused."
Concerns about the evacuation exercise should be addressed to Simon Fowle, the school bursar on 01622 845207 or at fowles@svs.org.uk