A carbon monoxide leak Monday at Finch Elementary School in Atlanta was caused by two maintenance workers who neglected to reopen a valve on the school furnace after inspecting it last Friday, Atlanta Public Schools superintendent Erroll Davis said Thursday.

The leak of the lethal, colorless, ordorless gas sent more than 30 students from the school to the hospital Monday. but All were treated and released.

The district discovered Wednesday after looking at school videotapes that the workers had done maintenance on the furnace last Friday, Davis said at a morning news conference.

“Neither of these two individuals came forward,” Davis said. “To say that is a disappointment is an understatement. We have now launched a full investigation of that.”

The district declined to name the employees, or say what if any action would be taken against them. School officials said they expect the school to reopen Friday since a temporary boiler has been installed and passed tests. The district is awaiting an official permit from the Atlanta Fire Department.