FORT MYERS, Fla. — Two teenagers were killed and at least 18 people were wounded early Monday when attackers raked a crowd with gunfire outside a nightclub here that had been hosting a party for young people, the authorities said.

Witnesses said multiple gunmen opened fire at about 12:30 a.m. outside the nightclub, Club Blu, in a palm-tree-lined strip mall in this southwestern Florida city.

Officials refused to discuss a possible motive, but “this was not a terrorist act,” Dennis Eads, the interim Fort Myers police chief, said at an afternoon news conference. “We have three persons of interest in custody — I’m not going to go into who they are — and we are still looking for others.”

The club had advertised a beach-themed “swimsuit glow party,” meant primarily for teenagers, with live music, a $5 cover charge and no proof of age required. In a statement, the club’s owner said that the event had ended and that people were leaving when the shooting occurred.