The United States suffered the worst mass shooting in its modern history early Sunday morning when 50 people were killed and 53 injured in Orlando, Fla., after a gunman stormed into a packed gay nightclub, officials said.

The gunman was killed by a SWAT team after taking hostages at Pulse, a popular gay club, investigators said at an early morning news conference. He has been preliminarily identified as 29-year-old Omar Mateen, a U.S. law enforcement official said. A federal investigator in Florida told reporters earlier Sunday that the gunman may have been attracted to radical Islam.

The “lone wolf” gunman was armed with an assault rifle, a handgun and an unspecified “device,” said officials, who did not elaborate on why they suspected he may have been a radical Islamist.

“It appears he was organized and well-prepared,” Orlando Police Chief John Mina said, adding that the gunman was not from the Orlando area.

The new death toll was a significant increase over officials’ previous estimate of 20 dead, which had already been enough to place Pulse among the gravest gun massacres in the nation’s history. Now no other American mass shooting comes close to the lives lost in Orlando on Sunday morning. Not at Columbine High School in Colorado, where 13 people died in 1999; nor in Newtown, Conn., where 26 people were killed in 2012; nor at Virginia Tech, where 32 people were killed in 2007.

Medical officials said that many of the 53 wounded were “critically” injured and had yet to be reunited with their families.

“Just to look in the eyes of our officers told the whole story,” Mina told reporters. “You could tell that they were all shaken by the incident, by what they saw in the club.”

The attack also comes during LGBT Pride month, when celebrations are being held in cities around the country, including Los Angeles, and will go down as one of the deadliest attacks on LGBT people in the nation’s history.

The gunman targeted a venue packed with about 320 club-goers who were so absorbed in the party that some witnesses mistook the opening gunshots sometime around 2 a.m. as part of the music.

Witnesses said they heard at least 40 shots fired, and one witness said the shooting lasted the length of an entire song. As bullets tore through the crowd, men and women took cover by dropping to the floor and crawling for cover. Some apparently hid in the restrooms, including one man whose texts to his mother were broadcast over WFTV-TV:

“Mommy I love you”

“In club they shooting”

“He’s coming”

“I’m gonna die”

“He’s in the bathroom with us”

The fate of the young man, who was not identified by the station, was not immediately known. Just after the shooting, Pulse Orlando posted a note to its Web page that said, “Everyone get out of Pulse and keep running.”

Officials said that the gunman initially traded gunfire with an Orlando police officer who was working as a security guard at the club. The gunman retreated into the club and held hostages there for about three hours.

Dozens of emergency vehicles surrounded the scene as dazed men and women, some wounded, escaped from the area. The Orlando Fire Department called its bomb squad and hazardous material team to the scene after 3 a.m. Emergency workers were seen taking victims away from the scene in large trucks — in one case, an emergency worker was doing chest compressions on one of the victims.

After 5 a.m., a SWAT team decided to move in, and the gunman shot an officer in the helmet before the team shot and killed him, Mina said — adding that the officer’s helmet appeared to save his life, and he suffered only an eye injury.

Rosie Feba said she took her girlfriend to the club for the first time Saturday night. It was near closing time when the shooting began.

“She told me someone was shooting. Everyone was getting on the floor,” Feba said. “I told her I didn’t think it was real, I thought it was just part of the music, until I saw fire coming out of his gun.”

Feba and her girlfriend ran out of the club. On the way out, they saw a man who had been shot. Feba grabbed him. Others around her called 911. Some of the man’s blood stained the sleeve of her striped T-shirt. Feba and her girlfriend were unharmed, but shaken.

Javer Antonetti, 53, went to the club with his brother and was toward the back of the room when shots ring out. “There were so many, at least 40,” he said. “It was constant, like ‘pow, pow, pow.’ ”

He ran, but his brother, who was on crutches, was stuck inside until police rescued him unharmed. After Antonetti escaped, he noticed he had blood smears on his shirt. He said he doesn’t know how they got there.

News of the shooting spread quickly over social media, as friends around and outside Florida tried to reach their acquaintances in Orlando to see if they were OK.

“One of my friends was shot and another is missing. I’m just sitting here shaking,” tweeted Joshua Yehl, the editor of IGN Comics in Los Angeles, who said he’s from Orlando. He added shortly afterward: “Just saw my best friend’s mom on the news, crying that her son went to Pulse and is now missing, and his bf was shot multiple times.”

Footage uploaded to a Facebook page belonging to Anthony Torres showed emergency responders treating several victims in a triage area that had been set up in the street. In another video, emergency workers appeared to be loading a victim into the back of an ambulance.

“They are just pulling people out in stretchers loading them up,” said a caption to the video. “Omg please god let everyone make it — tonight was supposed to be a fun night!! One more minute and we would of been shot or worse.. Thank god we got to our car in time.”

The FBI is assisting with the investigation.

The shooting came two nights after a brazen shooting at a different music venue in Orlando. On Friday night, singer Christina Grimmie was fatally shot as she signed autographs after performing at the Plaza Live theater in Orlando. Grimmie had gained fame as a YouTube performer and a contestant on “The Voice.”

Orlando Sentinel staff writers Hayes and Tziperman Lotan reported from Orlando, and Los Angeles Times staff writer Pearce reported from Los Angeles. The Associated Press contributed to this report.