HALIFAX -- Nova Scotia's justice minister says her department has launched an investigation into why a riot erupted at a youth jail, injuring five staff members.

Diana Whalen says she's concerned about the Sunday evening melee, and has directed staff to look into issues like staff training levels and the operations of the Waterville facility's health and safety committee.

"We need to look into it ... It will take about six weeks for us to get all of the information in and report back," she said after meeting with cabinet on Thursday.

She declined to comment on any details about the incident.

One 18-year-old man is charged with taking part in a riot and two counts of assault while carrying a weapon, while a 19-year-old man is charged with taking part in a riot and damage to property.

Two 17-year-old males were also arrested, and later returned to the youth facility in the Annapolis Valley.

Four injured youth workers were taken to hospital with what the Justice Department described as serious injuries and later released, and another staff member was treated at the scene by paramedics.

Whalen said there isn't a problem with staffing levels or overcrowding at the facility, as the staff-to-inmate ratios have increased in recent years -- and there is a children's mental health unity at the facility.

"This is the first time in over 30 years there's been an incident like that at Waterville. This is absolutely unprecedented and unusual. We want to learn lessons from it and find ways from allowing it to happen again," she said.

Her department released figures on Thursday indicating that in 2015, the facility admitted 237 youths, but as of this week there are only 23 residents. There are 81 staff members at Waterville.

However, the president of the Nova Scotia Government and General Employees Union, says his union is concerned about security levels at the facility.

"Youth workers should not fear for their lives when they go to work," said Jason MacLean.

"The health and safety of the women and men who work in the facility should be the priority of the employer and we have reached out to them looking for answers."

The union has said one of the workers was attacked, and when coworkers came to help, they were rushed and beaten by youth offenders who were able to open the doors in their rooms at the same time in a co-ordinated way.

MacLean, a corrections worker before becoming the union leader, said in an interview that if he'd known about the door system, he would have asked it be changed to prevent offenders from exiting simultaneously.

MacLean also said union members have told him that management had been alerted to the risk of keeping a volatile mix of young people together and of threats to staff in the days leading up to the incident.

The union president said female and male youth workers had received broken bones and serious cuts in the attack, but declined to provide further details.

MacLean said he plans to meet with the staff and offer support on Thursday evening.