HOUSTON – Recovery efforts have begun at the west Houston construction site where three workers were killed in a stairwell collapse Monday. Houston Fire Department Rescue Teams, two structural engineers and 60 contracting employees are assisting with recovery efforts at Marathon Oil’s new headquarters off Town and Country Blvd, according to officials. Firefighters said the collapse was isolated to one interior stairwell and the structural integrity of the building remains intact. Karla Valderas, the wife of one of the victims, said she’s still in shock her husband hasn’t returned home from work. READ: Recovery efforts underway to find bodies of 3 workers killed during stairwell collapseThe identities of the two remaining victims have not been released.