MADRID, Spain (AP) — A fire broke out in a nightclub in the southeastern Spanish city of Murcia on Sunday, killing 13 people and injuring several others, authorities said.

The fire started around 6 a.m. in the popular Teatre nightclub and quickly tore through the venue, according to Spain's state news agency EFE.

It was not immediately clear what caused the fire.

A video shared by Murcia’s fire service showed firefighters trying to control flames inside the nightclub. Police and emergency services worked to secure the interior of the club to avoid a possible collapse and were trying to locate and identify the bodies.

Officials said the death toll could increase.

The city council declared three days of mourning with flags flown at half-staff on public buildings throughout the region of Murcia.