Five people have been sent to hospital and 30 tenants are left displaced after a carbon monoxide leak forced the evacuation of a building in Saint John. Officials confirm one adult and two children remain in critical condition.

A carbon monoxide detector in the six-unit townhouse is credited with alerting the family and first responders to the leak at 1 Stephen Park around 10 p.m. Friday.

Tenant, Damien Burnett says he heard a bang on his window and a man telling him to open it.

Burnett says the man heard the carbon monoxide detector going off and called police.

"If no one came to the door, I would have gone to sleep that night,” says Burnett. “I was apparently poisoned from it."

He says police arrived within minutes and he saw officers carrying his neighbours out of their house without vital signs.

Officials say the two children were airlifted to the IWK Health Centre in Halifax, but a woman and her adult son from the adjacent unit at 3 Stephen Park were released from hospital after a brief period of observation.

The Canadian Red Cross is helping the mother and son with lodging, food and other basic needs until the leak is confirmed, repaired and air quality tests confirm the building’s safe. Officials say similar aid is being offered to the family from 1 Stephen Park.

Tenants of the remaining units (5, 7, 9, and 11 Stephen Park) were temporarily sheltered from the cold on Saint John Transit buses until the all-clear was given and they could either return home or other arrangements.

Platoon chief for the Saint John Fire Department Steve Voutour says carbon monoxide leaks can sometimes go undetected.

"In the case of a natural gas furnace, one of the byproducts is carbon monoxide, so if there is a leak in the system it could have leaked into the interior of the building and was undetected by the human beings,” says Voutour.

Investigators worked in and around the apartment's six affected units to ventilate the space throughout the day Saturday. Voutour says officers are working as quickly as possible to tenants back into their homes, but the building’s boiler will take time to be restarted.

The townhouse building is one of seven in a complex managed by the New Brunswick Housing Authority. In a statement, New Brunswick Housing says they’re working with residents to ensure they have other accommodations and they’re continuing to investigate the cause of the leak.

With files from CTV Atlantic’s Mary Cranston.