A 45-year-old Hagerstown man was seriously injured after an explosion Thursday, according to the Maryland State Fire Marshal. Authorities said an employee at Advanced Landscape and Construction on Salem Avenue in Hagerstown was trying to weld a partially filled 100-gallon fuel tank to a truck when it exploded around 11:47 a.m. Authorities said the fuel tank contained about 20 gallons of fuel when it exploded.