ORLANDO, Florida - At least fifty people are dead and many more badly hurt in the worst mass shooting in U.S. history. The suspected shooter has been linked to ISIS which has reportedly claimed responsibility for the attacks.

CBS News confirmed Orlando shooting suspect Omar Mateen called 911 and pledged his allegiance to ISIS during the attack.

Police say Mateen opened fire inside the crowded nightclub early Sunday, killing at least 50 people before dying in a gunfight with SWAT officers. Authorities are investigating it as an act of terrorism.

Mayor Buddy Dyer says 53 more are hospitalized, many in critical condition. He also said the shooter used an assault rifle on all those dead. Officials say one officer was shot and has injuries to his face.

Mateen was a Port St. Lucie, Florida resident who had been licensed as a private security officer in Florida. He was an American citizen born in New York.

The FBI investigated him in 2013 and 2014 but did not find enough evidence that he constituted a threat at that time, according to an FBI agent in Florida.

Orlando Mayor Buddy Dyer said at a press conference Sunday that a state of emergency has been declared after what authorities have indicated may have been a "lone wolf" style terror attack.

Orlando Police Chief John Mina said the suspect was found with a handgun and an AR-15-style assault rifle.

Medical officials say there is an urgent need for donations of O Negative blood, O Positive blood and AB Plasma. To find a donation center visit www.oneblood.org or call 1.888.9Donate. Donors should be healthy, age 16 or older and weigh at least 100 pounds.

A U.S. intelligence source told CBS news senior investigative producer Pat Milton that Islamic terrorism is being investigated as a possible motive in the shooting due to several indicators including the style of the attack, which had similarities to the attacks in Paris in November 2015 and Brussels in March. The gunman put up a protracted gun battle with police and was heavily armed, the source said.

Robots are now analyzing a device the gunman had strapped to his body to determine if it was a real explosive device or a fake made to look like a real device, the source said.

Mina said the incident at Pulse nightclub began around 2:20 a.m., when shots rang out and an officer already posted at the event engaged the gunman, whom Mina said was armed with an assault rifle, a handgun, and a "device" of some kind that officials believed was a threat. Mina said the gunman appeared organized and well-prepared for the incident.

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."

After the officer already there engaged the gunman, other law enforcement agencies and officials responded, and the gunman retreated into the nightclub and began a "hostage situation," Mina said.

SWAT officers eventually made the decision to attempt to rescue the hostages, and entered the club, where nine officers were involved in the fatal shooting of the gunman, Mina said. One officer was injured, and Mina said he believes the officer's life was probably saved by his Kevlar helmet.

How and when the numerous deaths occurred is still being investigated, officials said.

Mina and other law enforcement officials insisted there is no ongoing threat to the community.