Sgt. DB Gates of the King County Sheriff's Office says it appears the two-liter bottle was filled with some kind of volatile substance and exploded when the school bus driver walked by it.
"The staff member ... was hit by something that came out of the bottle and there's also a second adult who may have been exposed to fumes," Gates said. "Both were sent to the hospital."
She said neither of the two sustained serious injuries.
Kathryn Reith of the Lake Washington School District said the staff member who was hit was a school bus driver. She said the incident happened at about 7:30 a.m. outside Dickinson Elementary School and there were no children present at the time.
Dickinson Elementary and nearby Explorer Elementary both closed for the day to allow bomb experts to sweep the school.
Nothing else found during the search, and investigators determined the device that exploded was an acid bomb of a type commonly made by juveniles. It contained small pieces of wadded duct tape or foil that spewed out when it ignited.