Seattle police have arrested a 15-year-old and are seeking another suspect in a shooting on Sunday night that killed three people and injured as many as eight, including a toddler, at a food festival near the iconic Space Needle.

The 15-year-old will face initial charges of first-degree assault at a court appearance on Monday as investigators work to find the other suspect and determine which of them is responsible for the deaths.

Mayor Katie Wilson said the shootings at the Bite of Seattle festival were the third major occurrence of gun violence in the city in just a few days.

“This has been an especially tough week in Seattle,” she said.

Though crime is down in Seattle, city officials, including its new mayor, have struggled to respond to the city’s ongoing public safety problems in a few specific neighborhoods. Those problems include gang violence, drive-by shootings and human trafficking.

Ms. Wilson said Monday that investigators believed two people exchanged shots outside the armory building at Seattle Center, a tourist hub. They believe eight people were caught in the crossfire, including a 2-year-old boy whose injuries were not life-threatening.

Two adult men died at the scene, and an adult woman died later at a Seattle hospital.

Little is known about the second shooter, including whether that person is among the dead, authorities said.

“There is a second shooter,” Chief Shon Barnes said. “We are trying to identify who that person is. Our officers need your help.”

The city’s closed-circuit video surveillance system, the subject of intense debate when it was installed, does not extend to Seattle Center. Police investigators are using private camera footage and asking witnesses to turn over any cellphone video of the shooting or the moments afterward.

In April, gunfire disrupted the mayor’s announcement of new investments in meals and child care outside a community center southeast of the downtown area. Multiple bullets struck the building about 50 yards from where the mayor, community advocates and journalists had gathered for the event. No one was injured and the police said it was not a targeted attack. The mayor’s young daughter was at the event and was hustled to safety along with Ms. Wilson.

Much of the recent attention on violence and the debate over how to stop it has centered on a stretch of Aurora Avenue, a busy north-south corridor where drive-by shootings, open-air drug markets and commercial sex have challenged city leaders for years. Police have attributed the problems along Aurora Avenue to gangs and organized crime networks.

Last week, three people were injured in an early morning drive-by shooting just off Aurora Avenue north of downtown. Over the weekend, residents in nearby neighborhoods reported multiple bursts of gunfire, including one incident in which stray bullets narrowly missed a home with an infant inside.

Over the weekend, two bystanders were injured during a late-night shootout in the Capitol Hill neighborhood, a nightlife hub. Police have said they are investigating the Friday night violence as a gang-related incident.

Chief Barnes said Seattle was suffering from a spate of gun violence tied to “small groups that appear to be at odds with each other.”

Ms. Wilson said Monday that she planned to launch a renewed gun violence reduction strategy, with a special emphasis on youth violence prevention.

Though the mayor has approved expanded use of security cameras in the neighborhoods most affected, frustrated residents have resorted to building homemade barricades to deter through-traffic and the crime they believe accompanies it.