Forty-nine people were killed and 53 others wounded when a heavily armed gunman opened fire and seized hostages at a popular gay nightclub in Orlando, Florida -- the worst terror attack on US soil since September 11, 2001.
Here is what we know so far:
Who was the gunman ?
Law enforcement identified the gunman, who died in a shootout with police, as Omar Mateen, 29.
A US citizen, Mateen was born to Afghan parents in New York and resided in Port St Lucie, Florida, about two hours south of Orlando by car.
The FBI said it had previously investigated -- and cleared -- Mateen after he made inflammatory terror-related comments to co-workers and for suspected ties to an American suicide bomber.
Mateen made three 911 calls during Sunday's massacre and told the dispatcher he pledged allegiance to the Islamic State group, according to FBI Director James Comey.
The IS group claimed responsibility for the attack on Monday, calling Mateen "one of the soldiers of the caliphate".
But US investigators are still piecing together his motives, as he also claimed solidarity with the Boston bombers and a Florida man who carried out a suicide bombing in Syria for the Al-Nusra Front, which is in conflict with the IS group.
Comey said those facts added "a little bit to the confusion about his motives".
Law enforcement recovered a handgun, an AR-15 assault rifle and an unknown number of additional rounds from Mateen.
The gunman sprayed the club with bullets and took hostages in a three-hour standoff that ended when police stormed the venue.
Police chief John Mina said the authorities made that decision because they feared "further loss of life was imminent" among those still trapped in the venue.
What was his target?
Pulse is a popular club known on the Orlando LGBT party scene for its drag shows. It is not known exactly how many people were inside at the time of the attack, but the club was packed with Saturday night revelers who came to watch a drag queen contest on a special Latin music night.
Co-founded by a woman whose brother died in 1991 after battling HIV, the nightclub described itself as "more than 'just another gay club".
Barbara Poma and a friend opened the club in 2004 to keep her brother's spirit alive, naming it in honor of his heartbeat.
It has become a flagship establishment for the LGBT community in Florida and further afield, and forms part of a vibrant activist scene that aims to promote awareness of gay rights.
Was this an anti-gay hate crime?
US President Barack Obama called the massacre "an act of terror and an act of hate".
The suspect's father told NBC News his son may have been motivated by homophobia and not by extremism.
Seddique Mateen recalled a recent incident in downtown Miami when his son became angry after witnessing a gay couple embracing.
The suspect's ex-wife, who divorced him in 2011, told reporters he had been violently abusive to her and was not especially religious.
The Pulse attack coincided with Gay Pride month in the United States, with festive marches and events held across the country, including in Orlando last week and into the weekend.
Who are the victims?
The authorities now have identified all 49 victims.
Most of the victims were men ranging in age from 19 to 50, with many in their 20s and 30s.
Officials are publicly identifying victims only after their families have been notified.
Based on their names, most of the victims appeared to be of Hispanic heritage.
Police in the Los Angeles area on Sunday arrested a man who said he planned to attend the city's Gay Pride parade and was found with multiple weapons, ammunition and bomb-making materials in his car.
Police initially said he had expressed intent to "harm" those at the parade.
Later, they said investigators were trying to establish his intentions and that he had not referred to doing any harm, just to attending the event.
There was no known connection between 20-year-old James Howell and Mateen, authorities said