Investigators said Kinner was a temporary resident at the apartment complex, but had been asked to leave on Friday due to his behavior.
He returned on Saturday while a 3-year-old girl was having her birthday party a few doors down from where he was staying.
He picked his targets “at random,” police said.
Esrom Habte, 12, and Fathi Mahamoud, 11, were playing in the grass behind their apartment when the attack began, according to The Associated Press.
“It was kind of scary because we were outside,” Esrom told news media as his mother looked on. “He was chasing people and stabbing people.”
The children ran into an apartment and hid in a closet with other children until police told them it was safe to come out, the boy said.
“We saw him saying bad words and stabbing a grown-up really hard and a lot of times,” Esrom said.
Police found stabbing victims both inside the complex and in the parking lot; Kinner was arrested a short distance away.
The complex houses many refugee families, but Kinner is not a refugee, officials said.
“This incident is not a representation of our community but a single evil individual who attacked people without provocation that we are aware of at this time,” Bones said during a press conference Saturday night.
Bones said the attack resulted in the most victims in a single incident in Boise Police Department history.
“As you can imagine, the Wylie Street Apartment and our community is reeling from this attack,” he said.
Victim witness coordinators and counselors were being made available to the victims, their families and friends and other residents of the apartment complex, Bones said.
“Our hearts go out to the victims who are in the hospital tonight, please keep them and their families in your thoughts and prayers,” he said.
Boise Mayor Dave Bieter condemned the attack on Twitter.
“Last night’s horrific attack does not represent Boise,” Bieter wrote. “Please join me in praying for the injured and their families. We must come together to condemn this vile act.”
The Associated Press contributed to this report