Swedish investigators on Friday said they had released without charge a 16-year-old boy arrested as the alleged perpetrator of the shooting that left three dead — and arrested three other people.

Three people aged between 15 and 20 died in the shooting on Tuesday in the center of Uppsala, a university city some 60 kilometers (about 37 miles) north of the capital, Stockholm.

"During the time of the arrest, suspicions against the 16-year-old individual have weakened. He is no longer a suspect in the crime," lead prosecutor Andreas Nyberg said.

Two men aged between 25 and 35 were arrested for "incitement to murder" on Thursday, while a third, aged about 20, was arrested overnight Thursday to Friday on suspicion of murder, he said.

The motive for the murders is still unknown, but police are examining a possible connection to gang crime.

What we know about the killings

The three victims were found inside a hair salon, believed to be the scene of the crime.

Witnesses told police that two of the victims were customers sitting in the barber's chair getting their hair cut when the masked and dark-clad shooter shot them dead. They were hit by bullets in the head.

Swedish media have said that at least one of the dead had links to a criminal gang, but police have not confirmed those reports.

Prime Minister Ulf Kristersson deplored "a completely extreme act of violence" that "almost resembles an execution."

Sweden has been trying to stem a wave of shootings and homemade explosive attacks by rival gangs for several years, particularly over drug trafficking.

Uppsala is the home base for Sweden's two most notorious gangs, Rumba and Foxtrot. Their respective leaders and former associates, Ismail Abdo and Rawa Majid, are believed to be orchestrating operations from abroad.

Edited by: Zac Crellin