At least one person was killed, and five were severely wounded, during a Sunday morning shooting at a Swiss nightclub in the city of Aarau, local authorities said.
According to reports by the Swiss newspaper Blik, several people were killed when four gunmen entered the party, which was being held at the city's horse racing track.
The police confirmed that they were in pursuit of the attackers, while no official victim numbers had been released by authorities yet.
"Nighttime shots at event in Schachen. Several victims. Major police operation underway. Stay away from the area," said the police in a statement.
"Due to the ongoing major police operation, Aarau Swimming Pool will remain closed until further notice. Visitors are requested to stay away from Aarau Schachen," it added in a statement later.
Several people reportedly killed
According to the report by Blik, one of the partygoers said that people believed that the first shots fired were actually fireworks.
"We noticed that there was a shooting when someone yelled: 'Everyone down,'" he told the Swiss outlet, while he also detailed that his group had to hide under the DJ booth.
The report also noted that one person was killed, according to a witness, with one of the partygoers attempting to resuscitate a victim with several wounds.
In total, there were three fatal victims, according to Blik, with a total of four shooters present at the venue.
This is a developing story.