Three people have been killed, following a shooting at Castle Sports Complex swimming pool in Spalding, Lincolnshire.
East Midlands Ambulance Service said paramedics were told to approach the scene with caution and fought to save the three people, who had gunshot wounds.
Police urged members of the public to stay away from the area.
Britain has tough gun control laws and in recent years has seen a few dozen firearms deaths a year.
A spokesperson for the force said that an investigation into the shooting was ongoing and that there was "no indication" that this was a terrorist-related incident.
Lincolnshire police confirmed the three were dead including the suspected offender.
"There was a sound like a bird scarer going off", he said.
People reacted in shock when they heard the news about the shooting, with many taking to Twitter to share their disbelief.
Laura Spring, who works at the Woodlands Hotel on Pinchbeck Road, told the Lincolnshire Echo: "It is absolutely devastating".
In a short statement on its Facebook page, the Castle Sports Complex and Swimming Pool said it was closed "due to an incident in the local area".
"We apologise for any inconvenience caused".