NEW YORK - It's not clear when some Upper East Side residents can return home after a massive crack was found in their building.

The crack runs up most of the back wall of 300 East 96th Street.

About 30 people were evacuated from the complex Thursday, per an order from the Department of Buildings.

"We found a large crack in the rear wall of the building behind me that runs from the second floor all the way up to the roof. The building's owners were aware of the situation for the last couple of days. They brought in some engineers and started to repair it, but the tenants weren't happy I guess with the repair and called us for our evaluation," said FDNY Manhattan Borough Commander Roger Sakowich.

"I hope it's not going to go tumbling down because my concern is my good friend is right over here and I don't want him to get hurt," said one passerby.

Another building next door is under a partial vacate order.

The cause of the crack is under investigation. 

The Red Cross is helping residents find temporary lodging.