Seattle police say they now believe at least three people were involved in a fatal shooting at a food festival on Sunday.

Police initially said two suspects were thought to have been shooting at each other at the annual Bite of Seattle food festival at the Space Needle - one of the city's most popular tourist spots.

However, court papers relating to the arrest of one of these, a 15-year-old, on Monday revealed they now think there is a third. One is still being sought by police, while the other is thought to have been killed at the scene.

Three people were killed in the shooting and at least four others were injured, including a two-year old boy.

The teenage suspect waived his initial appearance in court on Monday, and he was remanded in custody pending trial, according to local media.

Ahead of the hearing, police revealed in court documents that they believe that three suspects may have fired shots during the "gunfight".

Court papers filed by the police say investigators are searching for "at least one other unknown suspect," according to local media. Seattle Mayor Katie Wilson, speaking at a news conference later on Monday, said: "Police continue to look for a third possible suspect."

The three people who died on Sunday were 19-year-old Junior Cee Niko Semo, Carlos Israel Sanchez Villalba, 44, and Ashley Whitehead, 56, according to the King County medical examiner. Police now believe that Semo was involved in the gunfight.

Two deaths were confirmed at the scene, and one person died later in hospital.

Two guns were recovered at the scene, police said.

Speaking on Monday afternoon, Assistant Police Chief Nicole Powell said that the shooting may have been "gang-related", but she cautioned that, "this is all preliminary information subject to change".

She added that the 15-year-old suspect and the 19-year-old who was killed are believed to be "associated with each other". She declined to describe their relationship.

Sunday was the final day of the annual three-day free food festival, which draws some 350,000 people each year.

Videos shared online showed people crouched on the ground, taking cover behind pillars and shelves of merchandise in the Space Needle's lobby during the shooting.

"At first, thought there were fireworks, and then everybody flooded and ran past us," Nick Bate told CBS news, the BBC's US partner.

Leanne Coles another witness told the station she used the Space Needle to try to keep track of where she was.

"Everybody started running, and so we just ran too, and we ran through the garden and we ducked and we decided to keep running with the crowd," she said.

Faith Adia Hunter told Associated Press that one of the shooters was near her and her friends when they heard the shots, sending them fleeing for shelter in the Seattle Children's Museum.

"We were right next to him when it started so we took off running," she said. "So, we took off running too into the building with the crowd."