(CN) – Fifty people were killed and another 53 were wounded at a gay nightclub in Orlando, Fla. early Sunday morning in what authorities are calling an act of terror and the worst mass shooting in U.S. history.

The gunman, Omar Mateen, 29, of Port St. Lucie, Fla., a sprawling town on the state’s Treasure Coast, opened fire on patrons at the Pulse nightclub shortly after 2 a.m. Sunday with an AR-15 assault rifle and a handgun, and then took scores of hostages before he was killed in a shootout with police.

“It appears he was organized and well-prepared,” Orlando Police Chief John Mina said during a briefing of the news media Sunday morning.

Mina said in addition to his firearms, Mateen was also carrying a “suspicious device,” although he declined to elaborate.

The FBI said he referred to the Islamic State in a 911 call he placed before the slayings.

Late Sunday, the City of Orlando released the names of seven victims. They were Edward Sotomayor Jr., 34, Stanley Almodovar III, 23, Luis Omar Ocasio-Capo, 20, Juan Ramon Guerrero, 22, Eric Ivan Ortiz-Rivera, 36, and Peter Gonzalez-Cruz, 22.

Florida Gov. Rick Scott responded to word of the carnage by stating “This is clearly an act of terrorism.”

“It should make every American angry,” Scott added.

The governor declared a state of emergency for all of Florida and the City of Orlando, and the two area hospitals that received victims from the shooting were for a time on lockdown Sunday morning.

At the White House, Lisa Monaco, Assistant to the President for Homeland Security and Counterterrorism, said President Obama had been briefed on incident and had directed the federal government “provide any assistance necessary to pursue the investigation and support the community.”

“Our thoughts and prayers are with the families and the loved ones of the victims,” the White House said.

Later, the president addressed the nation, calling the shooting “an act of terror” and an “act of hate.” He ordered the flag be flown at half-staff at the White House, all public buildings and grounds throughout the United States, and at military installations, embassies and consular officers across the globe until June 16 to honor the attack victims.

Attorney General Loretta Lynch is cutting short a visit to Beijing for cybersecurity meetings with Chinese officials and returning to the United States to monitor developments in the nightclub shooting investigation.

Overseas, Pope Francis expressed “horror and condemnation” at the “homicidal folly” of the shooting.

Florida Department of Law Enforcement Special Agent in Charge Danny Banks told reporters at the scene that the mass shooting is being investigated as an act of terrorism.

He said that in the early stages of what will likely be a lengthy investigation authorities are looking into whether this was an act of domestic or international terror, and if the shooter acted alone or with accomplices.

Witnesses who were inside the nightclub at the time of the shooting said the gunfire seemed to erupt out of nowhere and continued unabated for several minutes.

Pulse Orlando posted on its Facebook page around 2 a.m.: “Everyone get out of pulse and keep running.”

According to eyewitnesses, more than 100 people were inside the club when the shooting began.

A bouncer knocked down a partition between the bar and dance floor and an area in the back where only workers are allowed. People inside were able to then escape through the back of the club.

Police Chief Mina said a police officer working as a security guard at the club exchanged fire with Mateen as he was entered the club, guns abalze. Mateen continued firing and took at least 30 people hostage.

At 5 a.m., authorities ignited a device outside the club to distract the gunman, and then sent a SWAT team in to rescue the hostages. Mateen died in a gunfight with those officers.

Mina said one officer was shot in his Kevlar helmet during the gun battle, but received only minor injuries.

In the aftermath of the shootout, several police vehicles began ferrying shooting victims to Orlando Regional Medical Center.

“This kind of tragedy takes a toll on everyone, even law enforcement officers,” Mina said.

He said officials are in the process of searching the suspect’s vehicle, a black minivan parked at a carwash next door to Pulse. Police are using robots to aid in the search.

Orlando Regional Medical Center, the area’s only Level 1 Trauma Center, activated its Mass Casualty Incident Plan and brought in an additional six trauma surgeons. Many of the injured are in critical condition, said Dr. Michael Cheatham. A call for blood donations people out in droves and officials are now asking for water donations for the people waiting in line.

Orange County Sheriff Jerry Demings said the search for explosives in the building is hampering efforts to recover the bodies and it will likely be several more hours before they can start doing so. Bomb units from Orange County Sheriff’s Office, along with Orlando Fire Department are working together to sweep the premises, “We have all come together in unity,” Demings said.

State and Federal law enforcement officers joined local police on the Treasure Coast Sunday afternoon as they searched two homes associated with Mateen.

The first is an apartment that is believed to be his most recent known address, in the 2500 block of South 17th Street in Fort Pierce, Fla.

The second location, reportedly where he received mail, is his parents’ home on Southwest Bayshore Blvd. in Port St. Lucie.

The FBI, the Florida Division of Law Enforcement, St. Lucie County Sheriff’s Office and Fort Pierce Police Department are all at the apartment complex.

As the day wore on, authorities began to fill in what little is known about Mateen so far. He attended high school in Martin County, Fla. (just south of St. Lucie County), and received an associates degree in criminal justice technology in 2006 from Indian River State College.

He was employed by G4S Secure Solutions, a private security firm based in Hilton Head Island, South Carolina.

In a statement, company spokeswoman Monica Lewman-Garcia said “We are cooperating fully with all law-enforcement authorities, including the FBI, as they conduct their investigation. Our thoughts and prayers are with all of the friends, families and people affected by this unspeakable tragedy.”

St. Lucie County court records indicate Mateen married Sitora Yusufiy of Port St. Lucie in 2009, but that the couple divorced two years later. He was also the father of a three-year-old son.

A search of Florida Department of Law Enforcement turned up no state criminal record for him. Mateen held the firearms license since at least 2011. It was set to expire in September 2017.

FBI agent Ronald Hopper said Sunday that Mateen had been interviewed by the agency in 2013 and 2014. Hopper said agents first investigated Mateen after he made inflammatory comments to co-workers alleging possible ties to terrorists.

Mateen was interviewed twice and, when investigators were unable to verify the details of his comments, the FBI closed the probe.

In 2014, the agency looked into potential ties connecting Mateen to Moner Mohammad Abusalha, the first American to carry out a suicide attack in Syria. Like Maten, Abusalha lived in Fort Pierce, Fla.

Hopper says agents determined that contact was minimal and did not constitute a substantive relationship or a threat at that time.

The massacre elicited a range of responses from the presidential candidates. Presumptive Democratic nominee Hillary Clinton has long called for expanding background checks to sales at gun shows and online purchases, and for reinstating a ban on assault weapons. Presumptive Republican nominee Donald Trump has said the existing background check system should be fixed, not expanded, and that assault-weapons bans do not work.

On Sunday, Clinton echoed President Obama in calling the violence “an act of hate,” noting that “the gunman attacked an LGBT nightclub during Pride Month.”

“To the LGBT community: please know that you have millions of allies across our country. I am one of them. We will keep fighting for your right to live freely, openly and without fear. Hate has absolutely no place in America,” Clinton said.

“Finally, we need to keep guns like the ones used last night out of the hands of terrorists or other violent criminals. This is the deadliest mass shooting in the history of the United States and it reminds us once more that weapons of war have no place on our streets,” she said.

Her Democratic challenger, Sen. Bernie Sanders of Vermont, also issued a statement, saying “At this point we do not know whether this was an act of terrorism, a terrible hate crime against gay people or the act of a very sick person, but we extend our heartfelt condolences to the victims’ families and loved ones and our thoughts are with the injured and the entire Orlando LGBTQ community.”

A much different tack was taken by presumptive Republican presidential nominee Donald Trump, who commented on the incident through his Twitter account.

“Horrific incident in FL. Praying for all the victims & their families. When will this stop? When will we get tough, smart & vigilant?,” Trump tweeted early Sunday morning.

An hour later he tweeted: “Appreciate the congrats for being right on radical Islamic terrorism, I don’t want congrats, I want toughness & vigilance. We must be smart!”

Then, as President Obama addressed the nation, Trump tweeted: “Is President Obama going to finally mention the words radical Islamic terrorism? If he doesn’t he should immediately resign in disgrace!”

Investigators continue to saw they are investigating possible ties between Mateen and terror organizations like the Islamic State, but have made no definitive connection or announced one publicly.

There has been no claim of responsibility for the attack on jihadi forums, the authorities said.

Imam Muhammad Musri, president of the Central Florida Islamic Society condemned Sunday’s act of violence. “I’m here to today to stand as a faith leader with our law enforcement community and our city leadership in this hour of horror that was brought upon our city. Please call the FBI and share what you know.”

A family hotline has been established to give families information at 407-246-4357. The FBI is urging anyone who attended Pulse Orlando at any point Saturday night or Sunday morning to come forward as it puts its case together.

The incident followed the fatal shooting late Friday of 22-year-old singer Christina Grimmie, a former contestant on NBC’s “The Voice” talent competition, who was killed after her concert in Orlando by a 27-year-old Florida man who later killed himself.

Organizers of the 70th Annual Tony Awards, which are being held Sunday night in New York City, released a statement saying the celebration of Broadway’s finest will be dedicated to the victims.

“Our hearts are heavy for the unimaginable tragedy that happened last night in Orlando,” the statement said.

In a chilling development in light of the day’s events, a heavily armed man arrested in Southern California early Sunday told police he was in the area for West Hollywood’s huge gay pride parade, Los Angeles Mayor Eric Garcetti said.