President Barack Obama called Sunday’s attack on a gay nightclub in Orlando, Florida, that killed at least 50 people “an act of terror, an act of hate.”

“We grieve the brutal murder, the horrific massacre, of dozens of people,” Obama said at the White House.

About 50 people were killed and 53 were hospitalized, many in critical condition, after a gunman opened fire inside the Pulse nightclub early Sunday, police said.

Federal officials identified the gunman as Omar Mateen, an American citizen living in Port St. Lucie, Florida. Mateen, who was born in New York, died during a police raid of the building, officials said.

“He was a person filled with hatred,” Obama said.

Sunday’s attack was the worst mass shooting in American history. The 2007 shootings at Virginia Tech killed 32 people, while 26 people died in the 2012 attack at Sandy Hook Elementary School in Newtown, Connecticut.

Officials were investigating the attack as an act of terrorism. Rep. Adam Schiff, D-Calif., who is a member of the House Intelligence Committee, said in a statement that law enforcement officials told him Mateen had pledged allegiance to the Islamic State terror group. No group had claimed responsibility for the attack before Obama spoke.