Swiss police said they had arrested a 43-year-old suspect in relation to a deadly shooting at a rave in the town of Aarau at the weekend.

A 22-year-old Italian woman was killed and five people were injured in the early hours of Sunday morning at the event,held at a covered horse racing track.

Police launched a massive operation and just before 1am on Monday arrested a 43-year-old Swiss man on suspicion of carrying out the attack, they said.

"Investigations into the exact circumstances of the incident, as well as its background and possible motive, are currently under way. At this stage, it is assumed that a single perpetrator was involved," police in the canton of Aargau said in a statement. They gave no further details.

Police on Sunday said they were investigating three possible motives including a mass killing, an act of terrorism or an unknown criminal motive.

Cartridges found at the scene indicated two calibres of bullet, possibly from a pistol and an assault rifle, police said.

The Swiss newspaper Tages-Anzeiger said cartridges found resembled those used in the Russian AK-74 assault rifle and its variants.

Police declined to comment on the report.

The 22-year-old Italian who died was named as Maria Spinelli by Italian Prime Minister Giorgia Meloni.

Swiss President Guy Parmelin offered condolences to the family of the victim.

“In these difficult hours, my full sympathy goes to all people affected by this tragic event,” he said on social media.

A 34-year-old witness, who asked for anonymity, said he heard shots shortly after the music stopped at 2.30am on Sunday (local time), which he initially mistook for fireworks.

Some took cover behind a DJ booth and one woman hid behind a refrigerator, but was hit in the back by a bullet and bled to death despite efforts to revive her, he said.

“It’s just incredibly tragic. She was young,” he said. “That something like this could happen in Aarau, at a local event, is simply unimaginable. I still can’t really process it.”

The organisers’ website advertised the event as a techno party at a covered horse racetrack, with 3500 people expected to attend.

Organisers said they were shocked and heartbroken by the incident.

“A rave should always be a place of joy and love, never a place where people are killed, injured, or left in fear,” they said.

Switzerland has one of the highest rates of gun ownership in Europe, although violent shootings are rare and it approved tighter gun control rules in 2019.