At least four people were killed after a gunman opened fire during a music festival in Mexico’s Playa del Carmen city on Monday. A dozen others were injured in the shooting, which took place outside Blue Parrot nightclub on the last day of the BPM music festival that attracts swarms of foreign tourists. Festival organisers said three of the four dead were security personnel, The Guardian reported.

“We are overcome with grief over this senseless act of violence and we are cooperating fully with local law enforcement and government officials as they continue their investigation,” the BPM team said in a statement on Facebook. They added that they had been working to ensure public safety and security for all visitors.

The shots heard were initially mistaken as fireworks, before word spread that a “lone gunman” had been spotted.