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.

A fatal mass shooting in Glendale was isolated, police said. And it's one of dozens in the U.S. so far this year. But reputational concerns remain.