The latest detainee was arrested on Saturday.
They have analyzed parts of the collected material, which led to suspicions against a third person, says police spokesperson Magnus Jansson Klarin.
The person, who is over 18 years old, is detained on probable grounds suspected of murder.
Two people under the age of 18 have been detained since earlier. Both are reasonably suspected, a lower degree of suspicion.
The police have increased their presence in Gävle during the day, both for security and crime prevention reasons, according to Magnus Jansson Klarin.
It was at 5 pm on Friday that the alarm came in about a violent crime having occurred in the Öster district.
The police were initially very secretive about the incident and only wanted to say that one person had been taken to hospital.
It wasn't until 10:30 pm that the police confirmed that one person had been shot to death.
