VALLEJO (AP) — Two maintenance workers at Vallejo High School were seriously injured Friday in an explosion inside a classroom that was under construction, authorities said.

The workers were treated by Vallejo Fire Department personnel and taken by ambulance to a hospital, the Vallejo Fire Department said.

Officials at the Vallejo School District initially said a staff member suffered a burn injury to a hand during the explosion.

Vallejo High School was evacuated after the blast around 8 a.m. Friday. Students were not near the classroom, and no students were harmed, school officials said.

The blast happened in an unoccupied classroom that was undergoing a routine check following a construction repair project, they said.

The cause of the blast was under investigation. Pacific Gas & Electric Co. spokesman Paul Moreno said the utility did not detect any leaks or other problems with its gas lines leading to the school.

Authorities later gave the school the all-clear and classes resumed.