A 14-year-old and 18-year-old were killed in the attack.

Five people were detained by police, but it's unclear if any were believed to be a shooter. Neither the shooter's identity nor motivation was clear. Authorities are not sure if one shooter or multiple shooters were involved. The attack apparently occurred at a teen event billed as a "Swimsuit Glow Party" at Club Blu in Fort Myers, local media reported.

The shooting is "definitely not terrorism," police told Fox News' Heather Nauert. The incident appears to have been gang-related, a law enforcement official told Fox News' Matt Dean.

The violence at Club Blu erupted about 12:30 a.m. Monday, and there were two active crime scenes, Mulligan said. Several hours later a street in the area remained closed as police investigated.