Police in a Phoenix suburb say three people, including a teenager, were killed and five others were injured after a shooting at a crowded concert outside a restaurant.

The fatal shooting Sunday night at a restaurant and event venue in Glendale, Arizona, was an “isolated incident that involved individuals that were known to one another,” the city manager’s office said in an update Monday.

NBC affiliate KPNX of Phoenix reported that the restaurant El Camarón Gigante was holding a Cinco de Mayo party at the time.