DENVER (AP) — Authorities say four students suffered burns and at least one had serious injuries from a fire in a Denver high school chemistry laboratory.

The fire broke out Monday at SMART Academy, a charter high school in southwestern Denver. Denver Fire Department spokesman Mark Watson says the cause was under investigation, but the fire appears to have been accidental.

Watson says at least some of the students had burns to their faces and upper chests.

He says the fire appeared to have burned itself out and didn’t spread beyond the lab.

The blaze set off school alarms, prompting an evacuation. Watson says students have returned to classes.

A school spokesman says a teacher was in the lab at the time. Firefighters received the alarm shortly before 8 a.m.