A 43-year-old Swiss man was arrested Monday on "strong suspicion" of being behind a rare deadly gun attack at a rave in Switzerland, police said.

Gunfire erupted in the early hours of Sunday morning as an annual rave was winding down at the racecourse in Aarau, capital of Switzerland's northern canton of Aargau.

The attack triggered an international manhunt for the shooter, who left an Italian woman dead and wounded five others, some seriously.

"A 43-year-old Swiss man was apprehended shortly before 1:00 am Monday (2300 GMT Sunday)," Aargau police said in a statement.

"The man was subsequently arrested on strong suspicion of having committed the crime.

"Investigations into the exact sequence of events, the background, and the possible motive are currently under way.

"At this stage, it is believed a lone perpetrator was involved."

In a statement earlier Monday, the cantonal police said that partygoers were preparing to go home when several shots were fired "at a group of people".

A 22-year-old Italian woman was killed. "She died instantly, despite attempts to resuscitate her," said the police, adding that five others were injured.

On Sunday, Italian Prime Minister Giorgia Meloni said two Italians were among those wounded.

However the Aargau police said on Monday that those hurt were four Swiss men and a German woman, aged between 24 and 31.

They were taken to hospital "with injuries ranging from minor to serious", police said.

The cantonal police have scheduled a press conference for Wednesday.

On Sunday, Aargau police chief Michael Leupold said investigators had found evidence of two different calibres of weapon, from a pistol or submachine gun, and an assault rifle.

Leupold said police were working on three hypotheses: "The first is a terrorist attack, the second a mass shooting and the third a simple criminal act, the motive for which remains unknown."

He said this could be "a settling of scores within a group, or something entirely different".

On Monday, the police urged anyone with video footage to come forward.

Shootings are rare in Switzerland, despite widespread gun ownership.

The rave organisers said they were "devastated".

"The insidious attack on the guests at the Aarauer Rave has broken our hearts," they said on their website.

"A rave should always be a place of joy and love. Never a place where people are killed, injured or left in fear." 

Late Sunday, Swiss President Guy Parmelin sent his condolences to the family of the Italian victim.

"My thoughts are also with all those who have been injured; I wish them a speedy and full recovery," he said on X.

"My deepest sympathy goes out to everyone affected by this tragic event."

Swiss Justice Minister Beat Jans met German Interior Minister Alexander Dobrindt in southern Switzerland on Monday.

Jans said the Swiss authorities were "receiving support" from Berlin, praising the "strong cooperation" between police forces.

© Agence France-Presse