The man, whom police identified as Timothy Kinner, 30, was arrested in connection with the frenzied attack Saturday night.
The suspect's motive was "vengeance" for being asked to leave an apartment in the complex due to his behavior, Boise Police Chief Bill Bones said during an emotional news conference Sunday afternoon.
"Our victims are some of the newest members of our community. They're victims from their past homes who have fled violence from Syria, Iraq, and Ethiopia," Bones said.
He said there was no evidence this was a hate crime.
The rampage occurred at the Wylie Street Station apartments, a low-income complex where many refugees have been settled.
Bones said Kinner is from Los Angeles and had been invited to stay for a few days at the apartment of a renter who had shown him compassion, Bones said. But due to his disruptive behavior, Kinner was asked to leave the apartment on Friday, the chief said.
"He returned last night to exact vengeance, not just on those whom he had been [staying] with because they were not at the apartment, but at any target that was available," Bones said.
The stabbing spree unfolded around 8:46 p.m. when Kinner returned to the apartment where he had been staying and found nobody home, Bones said.
"The tragedy was that a 3-year-old little girl was having a birthday party just a few doors down from where Kinner had been staying," Bones said. "Kinner attacked, targeting the children initially. The 3-year-old girl, whose birthday it was, was one of those seriously injured."
He said Kinner then proceeded to allegedly stab other children attending the birthday party, two 4-year-olds, a 6-year-old, an 8-year-old and a 12-year-old. He then allegedly knifed three adults who tried to stop the attack, Bones said.
"Our suspect is a man with an extensive criminal record spanning multiple states, who has spent time in prison, and whose past criminal violations do include violence against others," Bones said. "He is an American. He's not a refugee."
As part of former President Barack Obama's refugee resettlement program of 2012, Boise has become a magnet for refugees from far-flung parts of the world.
Boise Mayor Dave Bieter said the attack "does not represent Boise" and he asked citizens of the community to keep the victims in their prayers.
"Our city has long stood as a welcoming city -- a place of safety and kindness for those fleeing violence and oppression in their homelands," Bieter said in a statement Sunday. "The senseless acts of one disturbed person does not change that. As Boiseans, we come together not only to condemn this vile act, but also to embrace those among us seeking refuge from tyranny, suffering and war. It is who we are and who we will continue to be."
Kinner was on Sunday being held on nine counts of aggravated battery and six counts of injury to a child, police said.
Four of the victims suffered life-threatening injuries, police said.
"My deepest thanks go to the medical team that pulled off the miracle to make sure that each of those faces has seen this morning's sunrise," Bones said of the wounded. "Lives do remain in danger. Some of the victims will be forever scarred by the medical injuries that they have received. They received life-changing injuries."
Bones said the victims were randomly targeted and had no prior connection of Kinner.
He said police arrived at the scene within four minutes after receiving calls of a man with a knife attacking victims at the apartment complex. He said Kinner was found walking away from the complex and arrested without a struggle.
The officers found the stabbing victims in the apartment where the birthday party had been occurring and in the parking lot. All nine victims were taken to a local hospital, and one seriously injured child was flown to a Salt Lake City trauma center for treatment.
"The faces of the parents struggling, the tears coming down their faces, the faces of the children in the hospital beds will be something that I'll carry with me for the rest of my life as will every first responder that was there last night," Bones said. "This was an attack against those who are most vulnerable, our children. It's untenable. It's unconscionable. It's pure evil in my mind."