Sixteen children and one adult were taken to hospital on Wednesday after they fell into the historic Fort Gibraltar during a school trip in the western Canadian city of Winnipeg, CBC News reported, citing emergency services.

The children are 10 and 11 years old, the CBC reported, adding that three of the children are in critical condition.

“Deeply concerned about the incident at Fort Gibraltar this morning involving a school group,” Winnipeg Mayor Scott Gillingham tweeted.

The city authorities have not yet provided additional details.

Fort Gibraltar in the province of Manitoba is one of over 1,000 protected national historic sites of Canada.

Tamim Aljafari, 10, said three fifth-grade classes were on a field trip to Fort Gibraltar on Wednesday and about 30 people were walking on a bridge inside the building when it collapsed.

“It happened to crack when we were on it, and then a lot of people just fell over,” he told reporters outside a children’s hospital in Winnipeg, according to CBC Canada.

According to him, the teacher and two students were under the bridge when it collapsed.

“I just didn’t know what happened and then I couldn’t breathe.”