ORLANDO, Fla. (Reuters) A gunman killed 50 people and injured 53 in a massacre at a gay nightclub early on Sunday (June 12), the city’s mayor and police said, in an attack U.S. authorities are described as a “terrorism incident.”

The shooter was identified as Omar S. Mateen, a man who a senior FBI official said might have had leanings toward 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.

A police officer working as a security guard inside the Pulse nightclub, which has operated in downtown Orlando since 2004, exchanged fire with the suspect about 2 a.m., police officials said.

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.”

President Obama ordered the federal government to provide any assistance necessary to local law enforcement officials investigating the shooting.

Obama was briefed on the shooting by Lisa Monaco, assistant to the president for homeland security and counterterrorism, the White House said in a statement.