CHICAGO (CBS) — At least 50 people were killed and 53 were injured early Sunday in Orlando, Florida, when a gunman armed with an assault rifle opened fire and took hostages inside a popular gay nightclub.

It was the worst mass shooting in U.S. history.

READ MORE: Two Simeon High School Students, Both 15, Killed In Shootings Four Hours Apart

So what do we know about the shooting?

• A gunman armed with an assault-style rifle and a handgun opened fire inside the crowded Pulse nightclub around 2 a.m. After an officer already at the club engaged the gunman, and other law enforcement agencies responded, the gunman retreated into the club, and took hostages. SWAT officers ultimately made an attempt to rescue the hostages, and entered the club. The shooter died in a gunfight with police.

• At least 50 people were killed, and 53 people were wounded, many of them critically. One officer was injured, and Orlando Police Chief John Mina said he believes the officer’s life was probably saved by his Kevlar helmet.

• The suspected gunman has been identified as Port St. Lucie resident Omar Mateen. Officials are investigating suspected ties to Islamic terrorism. Sources say Mateen called 911 and pledged allegiance to ISIS during the attack. His father has said his son was anti-gay, and he doesn’t believe the attack was motivated by religion.

READ MORE: Chicago Weather: High Waves, Dangerous Conditions Along Lakefront

• Mateen legally purchased the two guns he used in the last few days.

• The FBI confirmed federal authorities twice looked into Mateen in the past, and contacted him three times, but ultimately concluded there was no reason to pursue an investigation.

• Mateen was a licensed private security officer, who held a firearms license since at least 2011.

• ISIS has claimed responsibility for the attack.

MORE NEWS: Englewood Semi-Truck Driver Found Crushed Between His Truck And Trailer

• 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 http://www.oneblood.orgor call 1.888.9Donate. Donors should be healthy, age 16 or older and weigh at least 100 pounds.