The shooter was identified as Omar S. Mateen, a man that a senior FBI official said might have had leanings towards Islamic State militants. Officials cautioned that the Islamist tie required further investigation.

The death toll given by Orlando Mayor Buddy Dyer to reporters made the attack the deadliest mass shooting in U.S. history, eclipsing the 2007 massacre at Virginia Tech university, which left 32 dead.

Images from the Orlando Pulse Nightclub shooting:

A hostage situation quickly developed, and three hours later a squad of officers stormed the club and shot dead the gunman. It was unclear when the gunman shot the victims.

“Do we consider this an act of terrorism? Absolutely, we are investigating this from all parties’ perspective as an act of terrorism,” said Danny Banks, special agent in charge of the Florida Department of Law Enforcement. “Whether that is domestic terrorist activity or an international one, that is something we will certainly get to the bottom of.”

(Additional reporting by Letitia Stein in Tampa, Colleen Jenkins in Winston-Salem, North Carolina, Chris Michaud in New York and Mary Milliken in Los Angeles; Writing by Frank McGurty and Scott Malone; Editing by Mark Heinrich)