About 900 tenants of an east-end Toronto apartment building will have to find a new place to live for the next month as city officials investigate an underground explosion that rocked the property Sunday.

A structural engineer and the Fire Marshal's Office (FMO) has deemed the highrise building at 2 Secord Ave. unsafe after a hydro vault blew up, completely destroying an electrical room, several walls and a number of parked cars.

Fire crews were called to the building, in the Danforth Avenue and Dawes Road area at 10:43 a.m. Sunday after someone noticed smoke in the underground garage. Nine firefighters were slightly injured when the vault blew, sending a ball of fire about 30 feet in the air.

The firefighters, who suffered from concussions, smoke inhalation and second-degree burns, are all expected to recover.

The investigation and repairs will take at least four weeks, according to the FMO. A hydro worker said it was likely an isolated incided as the hydro vault underwent an inspection in 2006 and that at the time, "everything was functioning properly."

In the meantime, the building is being considered unsafe because it is without water, electricity and fire alarms.

Many of the building's residents are seniors who don't have anyone they can stay with, said Janet Davis, the local city councillor.

"Many of the tenants have pets in the building or they don't have family or friends to go to so we're going to do what we can to take care of them," she told reporters at the scene on Monday.

About 80 people sought shelter at a local elementary school.

She said despite the circumstances, then tenants are "remarkably calm and cooperative."

"They have shown tremendous strength," she said.

Many of the tenants were forced to leave their apartments without their wallets or a change of clothes. People were supposed to be allowed in at some point today to gather their belongings but emergency officials decided not to let the tenants in until later this week.

The priority, in the meantime, is to find them housing, said Davis.

In the meantime, residents are being asked to register at Secord Public School, which has become the headquarters for information.

The city will provide updates on the situation on their website. The building owner has also set up an information telephone line for residents at 416-429-0969.

With a report from CTV Toronto's Matet Nebres, Naomi Parness and Roger Petersen