Seattle Mayor Katie Wilson and Seattle Police Department Chief Shon Barnes said during a Monday morning press conference that yesterday’s deadly shooting at Seattle Center was the result of two people shooting at each other and hitting bystanders.

Wilson said a 15-year-old is being held at King County’s youth jail for a firearm offense and first-degree assault, and will make a first appearance in court today at 1:30 p.m.

Chief Barnes said police are still searching for a second shooter, but have not yet identified who that person is. Barnes said a SWAT team searched the Seattle Center yesterday searching for a second shooter, but found no one.

Barnes said witnesses should report what they saw to SPD’s tipline, and Mayor Wilson said investigators are checking footage from Seattle Center security cameras.

As for a motive, Barnes said he didn’t “know if it’s gang activity, but certainly group activity.”

Barnes said while it is possible one of the victims of the shooting is the second shooter, there has been no confirmation of that, and the investigation is ongoing. Authorities have yet to publicly identify any of the victims.

Seattle Fire Department Chief Harold Scoggins says two men, aged 19 and 44, died on the scene, and a 56-year-old woman died of her injuries at Harborview Medical Center.

Two women, aged 40 and 39, as well as a 23-year-old man were transported to Harborview and have since been discharged. A two-year-old boy remains in Harborview in “satisfactory” condition.

A 27-year-old man also showed up to UW Medical Center’s Montlake campus last night with a gunshot wound, and was later discharged, though SPD is still investigating whether his injuries were related to the Seattle Center shooting.