Authorities are offering up to $5,000 for anyone who can provide information on the person who set a fire Wednesday at the Fernald Center. "This was an intentionally set fire causing extensive damage to the basement level of the Fernald Administration Building," the State Fire Marshal's office said in a statement. A security guard found the fire in an administration building just before midnight Wednesday and called 911. Waltham Deputy Fire Chief Rich Grant said the fire was under control about an hour after firefighters arrived. One firefighter was treated for what Grant called a minor breathing issue. After a joint Waltham police, fire and the State Fire Marshal arson investigation Thursday morning authorities said a human act started the fire.