Swedish police arrested four men in connection with a triple murder that shocked the country and have released a teen previously detained over the shooting, prosecutors said on Friday.

Three people aged 15 to 20 were killed in a shooting on Tuesday in broad daylight at a hair salon in central Uppsala, north of Stockholm.

The shooting shocked Sweden amid rising concern over gang violence, though police have not confirmed that the shooting was gang-related.

"Suspicions against the 16-year-old have weakened during his time in custody. He is no longer suspected of a crime," prosecutor Andreas Nyberg said in a statement.

On Thursday, authorities arrested two men suspected of "incitement to murder," while a third man in his 20s was arrested overnight on Thursday to Friday suspected of murder, the statement said.

A fourth man, around the age of 4,5 was arrested on Friday, also suspected of incitement to murder.

Swedish media have reported that at least one of the dead had connections to a criminal gang, though police have not confirmed those reports.

Prime Minister Ulf Kristersson on Wednesday called the shooting "an extremely violent act," saying it looked "like an execution."

Uppsala is the home base for two of Sweden's most notorious gangs, Rumba and Foxtrot.

Their leaders, Ismail Abdo and Rawa Majid, are both now believed to be orchestrating operations from abroad.

Sweden has struggled for years to rein in gang conflicts, which have led to frequent shootings and bombings across the country that have occasionally claimed innocent victims. (AFP)