SALEM – A Lynn man died Saturday after he was hit in the head by a 50-pound sand bag that plummeted from the roof of a building on Washington Street.Juan Alvarado, 39, had been working with Sosa Construction of Beverly to repair a roof on a three-story building, located at 60 Washington St., when the heavy bag dealt a crushing blow. He was taken to North Shore Medical Center where he was pronounced dead around 1 p.m.Alvarado was said to have been walking by the building when the bag, which was being hoisted up, somehow broke free and fell. Police say they believe his death was an accident. A witness was said to have frantically called police after he heard people pleading for help in Spanish.Alvarado was given CPR at the scene by a Salem officer and resuscitation efforts were made by fire and ambulance crews to no avail.An investigation into the incident is being handled by officials from the Federal Occupational Safety and Health Administration (OSHA), along with State and Salem police.