A DJ, who was due to perform until the scheduled end of the event at 02:00, said the incident occurred “straight after our set”, in a post on social media.

Another performer, DJ Tekibo, said he had lost a friend in the incident. He wrote on Instagram that he was “still trying to process what happened”.

Olivier Grein, whose firm Agency Events AG organised the event, said the company was “shaken” and would do all it could to support the police investigation.

An eyewitness told Blick that he “suddenly heard the gunshots” minutes after the music stopped at 02:30, initially believing they were fireworks.

He said he was told to hide by a man and a woman, the latter of whom was then shot.

Organisers had expected up to 3,500 people to attend. Police asked witnesses to submit photographs and video from the scene.

An area around the racing track, the town’s swimming pool and sports field has been cordoned off, with the public urged to stay away.

A large contingent of officers has also been deployed beyond Aarau “as a precaution”, Aargau Cantonal Police said.

An AFP journalist reported seeing armed officers deployed on the highway up to 30km from the site of the shooting.

Aarau, home to approximately 23,000 people, is less than 40km from the border with Germany, where local media reported police were supporting Swiss forces.