A Monday night "riot" at a Duluth youth treatment center was sparked by a single client striking a staff member in the face, authorities said Monday.

Before it was over, the situation involved approximately 20 clients, more than 30 staff members and 21 police officers from four agencies, according to court documents and a statement from officials at The Hills Youth and Family Services.

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Police arrested an 18-year-old male and three juveniles in the aftermath. Three staff members suffered injuries that did not require medical attention.

"I cannot say enough good things about the professionalism of the staff who were here Monday night, the police who responded to the call, and the clients who acted responsibly," Hills CEO Jeff Bradt said in a statement.

"Most of our clients did exactly what they were supposed to do and stayed in their residential units during the disturbance. Some clients' curiosity drew them into the hallways during the disturbance. The officers helped us redirect those clients to their units."

The incident happened around 9 p.m. at the residential treatment center, 4321 Allendale Ave., in the Woodland neighborhood. It started when a client became "agitated," spokesman Steve Greenfield said.

"After assuring staff he was now calm, he struck a staff person in the face," Greenfield said. "As another staff member helped the injured staff control the client, at least one other client joined the assault. As more staff arrived to assist staff dealing with the disturbance, some clients followed them there."

No other clients joined the assault, Greenfield said, but one did interfere with staff. Another tried to help the staff, while remaining clients stayed out of the scuffle, he said.

Concerned the incident could "get out of control," Greenfield said the building supervisor called the Duluth Police Department and also put a call for available staff. The 17 staffers on duty were joined by 15 off-duty employees, he said.

A total of 21 law enforcement officers from the Duluth, Hermantown and University of Minnesota Duluth police departments, as well as the St. Louis County Sheriff's Office, responded to the scene, according to court documents filed Wednesday.

"The responding police officers helped establish order and calm clients," Greenfield said. "The police then moved away from the residential units as quickly as they could, knowing that their presence was unsettling for some."

The four clients arrested were the two who allegedly assaulted staff, the client who allegedly interfered and another who allegedly "created a disturbance that may have precipitated the assault," Greenfield said.

The lone adult, 18-year-old Joshua Yumandan Ali, was arraigned Wednesday morning in State District Court in a gross misdemeanor charge of third-degree riot and a misdemeanor count of obstructing the legal process.

A criminal complaint states that police arriving on scene found Ali "yelling at staff before becoming aggressive" and "singled out one particular staff member."

"A group of nine surrounded the staff member while (Ali) repeatedly and threateningly charged at the staff member," the complaint said.

Ali allegedly failed to comply with the commands of officers and attempted to fight his way out of their control. It took several officers to finally subdue and handcuff him, the complaint said.

Records show that Ali, of Plymouth, Minn., was sentenced to the facility after being adjudicated in juvenile court for robbing a suburban Twin Cities restaurant and holding several employees captive at knifepoint in June 2017.

Ali's placement at the Hills was terminated following Monday's incident, court records show. He remained in the St. Louis County Jail, with a Hennepin County warrant issued as a result of the alleged violation of his probation in the earlier case.

Specifics on the other three arrestees were not available because they are juveniles.

The Hills, operating since 1971, offers residential and day treatment for youths with behavioral or mental health needs, including many who are court-ordered to be there. It was formerly known as Woodland Hills.

The facility currently cares for 70 boys and girls, ranging in age from 10 to 20, in its residential treatment center. Officials said the facility's role in treating vulnerable youths makes it "susceptible to disturbances and challenges."

"Many adults need help dealing with challenges they face," Bradt said. "The kids we serve are no different. Sometimes, even a young person who's making progress can become overwhelmed and lash out at whoever happens to be closest. That's what happened Monday night."