(CNN) -- An explosion that injured four people at a University of Missouri science lab Monday afternoon was caused by human error, authorities said.
Lab personnel failed to turn off a hydrogen tank during an experiment and the building concentration of gas eventually exploded, the Columbia Fire Department said after an initial investigation. The building's sprinkler system put out the fire.
"Glass and debris from the third floor Biochemistry Lab had rained down onto the sidewalk, courtyard and driveway to the building, fire officials said. "Seventeen third-story windows were blown out."
The injured were transported to University Hospital, officials said. Three of the victims had been released by late Monday. The fourth was being treated in the hospital's burn unit.
"The number of injuries was certainly reduced due to the campus being on summer break, with a significantly reduced number of people in and around this facility," fire officials said.