ORLANDO, Fla. – Sen. Bill Nelson says that ISIS has taken credit for the mass shooting at a gay nightclub in Orlando and police say that the gunman called 911 declaring allegiance to the Islamic State.

Police said the gunman, identified as 29-year-old Omar Mir Seddique Mateen, opened fire early Sunday morning at Pulse nightclub in Orlando, killing 50 people and injuring at least 53 more in what is the worst mass shooting in U.S. history.

Mateen called 911 around the time of the attack to pledge his allegiance to ISIS and he mentioned the Boston bombers, according to the FBI.

News 6 was on scene Sunday afternoon as local and federal bomb squad personnel were entering Mateen's Fort Pierce apartment. It's not clear what they found. Mateen, an American citizen born in New York, has been on law enforcement radar since 2013.

[MORE: Victims ID'd in mass shooting | WATCH LIVE: News 6 coverage | PICS: Authorities investigate | RAW VIDEO: 7:30 a.m. news conference | 10 a.m. news conference | Gov. Scott speaks | TIMELINE: Scroll below]

Officials said Mateen made the 911 call more than 20 minutes into the attack and had legally purchased two of the firearms, a handgun and a long gun, used in the shooting within the last week in Florida.

Mateen had been previously investigated by the FBI as possibly having ties to Islamic extremism. The investigations didn't find evidence to charge Mateen with anything, officials said.

The FBI said that Mateen was not under surveillance or the subject of an investigation at the time of the mass shooting.

Dyer also declared a state of emergency in the city of Orlando and Gov. Rick Scott issued a state of emergency after the shooting, which police are considering an act of domestic terror.

U.S. Sen. Bill Nelson said that there appears to be connection to radical Islamic terrorism, possibly ISIS.

“Our thoughts and prayers go out to the families of all the victims of this horrific attack in Orlando. I have spoken with the FBI and there appears to be a link to Islamic radicalism," Nelson said. "I have conferred with additional sources and they think there is likely a connection with ISIS. So as we all mourn the senseless loss of life and investigators continue to learn more, we must remain vigilant and remember that if you see something, say something.”

The shooting, which Orlando Police Department is calling "one of the worst tragedies," unfolded at Pulse, described on its website as the "hottest gay bar in Orlando," which is located at 1912 S. Orange Avenue near Kaley Street.

Florida Nightclub Shooting | Graphiq

In addition to the slayings, at least 53 other victims were taken to area hospitals for medical treatment, police said. There were an estimated 320 people in the club when it opened Saturday night.

Barbara Poma, who owns Pulse, released a statement late Sunday afternoon.

"Like everyone in the country, I am devastated about the horrific events that have taken place today. Pulse, and the men and women who work there, have been my family for nearly 15 years. From the beginning, Pulse has served as a place of love and acceptance for the LGBTQ community. I want to express my profound sadness and condolences to all who have lost loved ones. Please know that my grief and heart are with you."

Orlando police said at a 10:30 a.m. news conference that they had been searching and investigating the suspect's vehicle. They also asked anyone who attended Pulse nightclub and left before the shooting to call the FBI at 800-CALL-FBI.

Orange Avenue will be closed from Kaley Avenue to Grant Street until further notice, police tweeted.

Authorities asked for patience as more information comes out.

"There's blood everywhere," U.S. Rep. Alan Grayson told reporters. "There's an enormous amount of evidence to be collected."

I'm arriving to #orlandoshooting suspect's home. Police have entire apartment building blocked off. @news6wkmg pic.twitter.com/ctkiwdnqkS

— Troy Campbell (@TroyLeeCampbell) June 12, 2016

Witnesses detail shooting

"Everybody was just running out through the back (of the club)," a witness told News 6.

A clubgoer said that he saw one person holding what he called an assault rifle and firing rounds inside Pulse. Police said the gunman had an assault-type rifle, a handgun and some sort of device strapped to himself.

Another clubgoer said he heard at least 40 shots fired. Another witness said about 100 rounds were fired.

"I saw blood everywhere," another clubgoer said.

Orlando Police Chief John Mina said an officer in full uniform was working extra duty at the club and heard reports of shots fired. He engaged the shooter, who went back inside the club and created a hostage situation, Mina said.

About three hours later, Mina said a decision was made to rescue the hostages, and the shooter was killed in a gunfight with officers.

There were 11 police officers involved in the shooting, one of whom was injured. Three Orange County Sheriff's Office deputies also fired their weapons. All officers have been relieved of duty and the Florida Department of Law Enforcement will investigate the shooting.

"His Kevlar helmet saved his life," said Mina, adding that the officer's injuries were not serious.

The victims

Orlando Health, which owns Orlando Regional Medical Center, said it has 44 patients at the hospital from the shooting and that nine patients have died since they were admitted. The hospital is working on reuniting patients with their families.

The hospital performed 26 operations Sunday on nightclub shooting victims on wounds determined to be from an assault rifle to the trunk, extremities and abdomen.

Seven victims have been identified in the shooting.

Scott asked for a nationwide moment of silence, which was observed at 6 p.m. Sunday.

PULSE NIGHTCLUB SHOOTING: Keeping you updated from inside News 6 #PrayForOrlando Posted by News 6 WKMG / ClickOrlando on Sunday, June 12, 2016

More than 100 law enforcement officials were called to the scene, and victims were taken to nearby Orlando Regional Medical Center. Video also showed clubgoers carrying wounded victims to the hospital.

Orange County Sheriff Jerry Demings said they have been unable to remove all the victims due to ensuring the safety of the building.

"We are a united community and what we saw last night does not reflect what we feel in our hearts and souls," said Orange County Mayor Teresa Jacobs.

Social media erupted with reports about the shooting, saying there were mass casualties, which Orlando police later confirmed.

We can confirm this is a mass casualty situation. Support from local/state/federal agencies. We expect to brief media shortly. — Orlando Police (@OrlandoPolice) June 12, 2016

A message posted to Pulse's Facebook page around 2 a.m. said, "Everyone get out of pulse and keep running."

Hours before the shooting, the club urged partygoers to attend its "Latin flavor" night.

After the shooting, Orlando police tweeted that people should not go near the area. Several surrounding streets were closed as authorities investigated the incident. Just after 5 a.m., a controlled explosion was conducted at or near the club, Orlando police said. About 15 minutes later, several rounds of gunfire were heard in the area.

Bomb dog searches ORMC emergency room following Pulse nightclub shooting. Posted by Mike DeForest - News 6 on Sunday, June 12, 2016

Orlando Regional Medical Center was placed on lockdown around 2 a.m. after receiving several gunshot victims.

If you believe an immediate family member may be in the hospital, please come to the main entrance of ORMC’s new north tower. ID required. — Orlando Health (@orlandohealth) June 12, 2016

"Only essential workers are being allowed access into the building," officials said.

Arnold Palmer Hospital and Winnie Palmer Hospital were also placed on lockdown, but the lockdown was lifted just after 8:30 a.m.

How you can help

Orange County firefighters said blood, especially Type O is in large demand in Orlando. You can find the One Blood information here and reach them by phone at 1-888-9Donate.

For emotional support, Orlando police said you can live chat online at HFUW.org or text zip code to 898-211 to connect via text. You can also dial 211 for the Information/assistance helpline for emotional support.

As of late Sunday night, a gofundme account dedicated to the victims of the shooting had raised more than $1,200,000.

The mass shooting took place less than 4 miles from where "The Voice" singer and YouTube star Christina Grimmie, 22, was shot dead at The Plaza Live in Orlando on Friday. The motive in that shooting remains a mystery.

Orlando police said the Pulse nightclub shooting was not related to the shooting at The Plaza Live.

No other details have been released.

Watch News 6 and stay with ClickOrlando.com for more on this story.

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