A California mass shooting that left three children and a young adult dead at a little girl’s birthday party was gang-related, according to the town’s mayor.

Family members were getting ready to cut the cake at a toddler’s birthday party when the gunfire started inside a banquet hall packed with relatives and friends over the weekend in California.

The victims, including children and adults, were believed to have been at a family gathering at a banquet hall when the shooting occurred, Heather Brent, a spokeswoman for the sheriff’s office, said a...