ORLANDO, Fla. (CBSNewYork/AP) — A gunman wielding an assault-type rifle and a handgun opened fire inside a crowded Florida gay nightclub early Sunday before dying in a gunfight with SWAT officers, police said. The attack left at least 50 people dead, making it the worst mass shooting in American history.

Pulse nightclub shooting: Approximately 20 people dead inside the club. — Orlando Police (@OrlandoPolice) June 12, 2016

The Islamic State of Iraq and Syria, or ISIS, claimed responsibility for the shooting, CBS News reports. The terror group’s news organization, Aamaq Agency, said the attack was “carried out by an Islamic State fighter.”

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CBS2’s Lou Young was in Orlando Sunday night. He said the area where the shooting happened — just southwest of downtown Orlando — was a wide avenue lined with strip malls, restaurants and stores. It would be a familiar scene to anybody who lives on Long Island or in New Jersey as part of our American topography.

But now, it is the scene of a new American tragedy.

The patrons of the Pulse nightclub first thought the gunshot sounds were part of a show. Then they saw people fall as the sounds kept coming, and blood was everywhere.

“So it wasn’t until you could start smelling that ammunition that people actually got the hint, and they heard it repeated (the gunshots) — and that’s when everybody dropped to the floor,” said witness Janiel Gonzalez. “And it was just complete and utter chaos.”

One man with a military assault rifle was behind the shooting.

Law enforcement sources told CBS News the gunman has been identified as Omar Mateen, 29, a U.S. citizen from Port St. Lucie. Mateen was born to Afghan parents. CBS News reports Mateen has no apparent criminal history and that authorities are investigating whether he had ties to Islamic terrorism.

“I want to say that there was anywhere from like 45 to like maybe 60 rounds being shot, and like as soon as he depleted one of them, he put another one in, and he was basically just swiping around — no specific purpose — kind of just shooting anybody and anything,” Gonzalez said.

CBS News Justice and Homeland Security correspondent Jeff Pegues reports that that Mateen called 911 just prior to the attack and pledged allegiance to ISIS. According to CBS News, Mateen identified himself to the 911 operator and also mentioned the Tsarnaev brothers, who were behind the Boston Marathon bombings in 2013.

Sources said Mateen had been on law enforcement’s radar in the last five years, and that he was not under investigation at the time of the shooting.

Ron Hopper, the assistant FBI special agent in charge, said that they first became aware of Mateen in 2013 when he made inflammatory comments to co-workers about possible terrorist ties. Hopper said the FBI investigated the matter and interviewed Mateen twice, but were unable to verify the substance and the investigation was closed.

Hopper added that Mateen came to the attention of the FBI again in 2014 due to possible ties with American suicide bomber Moner Abu Salha. Hopper said that they determined that the contact was “minimal and did not constitute a substantive relationship or a threat at that time.”

CBS News reported even early in the day that authorities had been “leaning towards Islamic terrorism” as a motive. Law enforcement sources said the shooting had similarities to the terror attacks in Paris and Brussels. Authorities are also investigating whether the shooting was a hate crime.

Names of some of the victims have been released by the City of Orlando as officials work tirelessly to identify individuals and notify family.

Among them are:

Edward Sotomayor Jr., 34 years old

Stanley Almodovar III, 23 years old

Luis Omar Ocasio-Capo, 20 years old

Juan Ramon Guerrero, 22 years old

Kimberly Morris, 37 years old

Eric Ivan Ortiz-Rivera, 36 years old

Peter O. Gonzalez-Cruz, 22 years old

Luis S. Vielma, 22 years old.

At least 53 other people were hospitalized, most in critical condition, officials said.

“I think we will see the death toll rise,” said Dr. Mike Cheatham a trauma surgeon at Orlando Regional Medical Center.

PHOTOS: 50 DEAD IN ORLANDO NIGHTCLUB ATTACK

“There’s blood everywhere,” Mayor Buddy Dyer said Sunday at a news conference, hours after the shooting.

Dyer said the shooter used his assault rifle on all those killed.

Police Chief John Mina also said the shooter had some sort of “suspicious device.” He said the suspect exchanged gunfire with an officer working at the club around 2 a.m., then went back inside and took hostages among more than 300 people inside.

Mina said Mateen had a handgun and an AR15 rifle.

Mina said that 11 officers exchanged gunfire with Mateen at the nightclub.

CBS News reports that robots are analyzing a device strapped to the gunman’s body to determine if it is a real or fake explosive device.

Around 5 a.m., authorities sent in a SWAT team to rescue the hostages, and the suspect then died in a gunfight with those officers.

CBS2’s Magdalena Doris reported the hostage standoff lasted for three hours.

Florida Gov. Rick Scott has declared a state of emergency for Orange County and called the shooting an “act of terror.”

“To take that number of lives is clearly an act of terror,” Scott said.

President Barack Obama called the massacre an “act of terror” and an “act of hate.”

“The shooter targeted a nightclub where people came together to be with friends to dance and to sing and to live,” Obama said. “The place where they were attacked is more than a nightclub — it is a place of solidarity and empowerment where people come together to raise awareness, to speak their minds and to advocate for their civil rights.”

Obama also touched on gun control in his statement.

“This massacre is a further reminder of how easy it is for someone to get their hands on a weapon that let’s them shoot people in a school, or in a house of worship, or movie theater, or in a nightclub,” the president said. “And we have to decide if that’s the kind of country we want to be, and to actively do nothing is a decision as well.”

Vice President Joe Biden also released a statement mourning the loss of the victims.

“They were our brothers and our sisters; our friends, neighbors, and loved ones,” Biden said in the statement. “In the coming days, we will learn more about these fifty souls and the lives they lived and the world they made better.”

Biden reiterated that the White House is closely monitoring the investigation.

“Jill and I offer our prayers and deepest condolences for all those affected by today’s horrific events. But our prayers are not enough to end these kinds of senseless mass shootings,” Biden said in the statement. “The violence is not normal, and the targeting of our lesbian, gay, bisexual, and transgender Americans is evil and abhorrent.”

Mir Seddique, Mateen’s father, told reporters that his son had anti-gay leanings and the shooting doesn’t have anything to do with religion.

Seddique said his son got angry when he saw two men kissing in Miami a couple of months ago and thinks that may be related to the shooting, WFOR-TV, CBS4 in Miami reported.

Seddique says: “We are saying we are apologizing for the whole incident. … We are in shock like the whole country.”

Pictures have emerged on social media of Mateen wearing NYPD apparel.

NYPD News said that “there is absolutely no affiliation at this time.”

Re: photos of the alleged #Orlando shooter wearing #NYPD clothing, there is absolutely no affiliation at this time." @NYPDChiefofDept — NYPD NEWS (@NYPDnews) June 12, 2016

Witness Shawn Royster spoke with Scott Pelley on the CBS Evening News Sunday. He said the night began with, “Happy faces; everybody was having a good time actually, and then, next thing you know, it was all like mayhem.”

Royster said he heard the gunshots for “a good minute… a good two, three minutes,” and went over a wall to escape the scene. He said he hid under a truck across the street until the gunfire was over.

Royster cried as he described what he heard.

“Screaming, yelling, they were like dragging bodies… just to get them out of the way,” he said. “Probably the worst fear I’ve ever felt in my entire life.

Mina Justice was outside the club early Sunday trying to contact her 30-year-old son Eddie, who texted her when the shooting happened and asked her to call police. He told her he ran into a bathroom with other club patrons to hide. He then texted her: “He’s coming.”

“The next text said: ‘He has us, and he’s in here with us,”‘ she said. “That was the last conversation.”

Jackie Smith, who was inside the club, said two friends next to her were shot.

“Some guy walked in and started shooting everybody. He had an automatic rifle, so nobody stood a chance,” Smith said. “I just tried to get out of there.”

Smith did not know the conditions of her friends and came out of the hospital and burst into tears.

Christine Leinonen drove to Orlando at 4 a.m. after learning of the shooting from a friend of her 32-year-old son, Christopher Leinonen, who was at Pulse and is missing.

“I don’t know where my son is. No one can tell me where my son is — if he’s been shot, if he’s dead,” Christine Leinonen said in sobs. “No one knows.”

She has not heard from her son and fears the worse.

“These are nonsensical killings of our children,” she said, sobbing. “They’re killing our babies!”

She said her son’s friend Brandon Wolf made it out alive by hiding in a bathroom and running out as the bullets flew.

Pulse Orlando posted on its own Facebook page around 2 a.m.: “Everyone get out of pulse and keep running.” Just before 6 a.m., the club posted an update: “As soon as we have any information we will update everyone. Please keep everyone in your prayers as we work through this tragic event. Thank you for your thoughts and love.”

Pulse describes itself as the “hottest gay bar in Orlando.” CBS affiliate WKMG-TV reported that it was “Latin Night” at the nightclub at the time of the shooting.

Jon Alamo said he was at the back of one of the club’s rooms when a man holding a weapon came into the front of the room.

The office for Mayor Bill de Blasio said that NYPD patrol and counterterrorism “resources are on alert and stand in protection of our city.” Increased patrols were to be dispatched in the days to come, particularly in heavily-trafficked areas and key LGBT community institutions.

Both @NYPDnews Patrol and Counterterrorism resources are on alert and stand in protection of our city. — NYC Mayor's Office (@NYCMayorsOffice) June 12, 2016

“We mourn with the people of Orlando and the LGBT community as a whole on the news that — once again — we have lost precious lives to the gun,” the mayor’s office tweeted.

We mourn with the people of Orlando and the LGBT community as a whole on the news that -once again- we have lost precious lives to the gun. — NYC Mayor's Office (@NYCMayorsOffice) June 12, 2016

The incident follows the fatal shooting late Friday of 22-year-old singer Christina Grimmie, who was killed after her concert in Orlando by a 27-year-old Florida man who later killed himself. Grimmie was a YouTube sensation and former contestant on “The Voice.”

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