An unexpected explosion during a Long Beach Fire Department training exercise left 11 people with minor injuries Tuesday morning, according to the LBFD.

The explosion happened just as a group of recruits was starting their first round of live-fire training at the Fire Captain David Rosa Regional Training Center at 2249 Argonne Avenue, the LBFD said.

Fire officials said the basement of a tower used in the training exploded unexpectedly. It’s not clear yet what caused the blast, but it was powerful enough that it broke windows and caused minor damage throughout the structure, fire officials said.

Everyone involved in the training was “evacuated from the structure and quickly accounted for,” the LBFD said in a series of tweets.

All the injuries appeared to be minor, but one fire recruit was taken to a local hospital for further evaluation, according to the LBFD.

“Other involved personnel will receive further evaluation by occupational medical staff to address any additional complaints or injuries,” officials said.

Training was suspended for the day and the center was closed. The department said operations will resume when it is safe to do so, although it is unclear when that will be.

The cause of the explosion is under investigation.

UPDATE: Second Street reopened after authorities investigate suspicious device