Omar Mateen and his wife, Noor Zahi Salman, visited Walt Disney World in April, the source says. Salman told federal authorities on Sunday that her husband had more recently been "scouting Downtown Disney and Pulse [nightclub] for attacks."
Unlike the four Disney World theme parks, Downtown Disney, which was recently renamed Disney Springs, doesn't have security and bag check before entry.
Metal detectors have been installed outside Magic Kingdom, Hollywood Studios, Animal Kingdom and Epcot since 2015 and guests are selected at random for screening.
Representatives for Walt Disney World did not immediately return phone calls seeking comment.
Despite being investigated by the FBI twice before Sunday's tragedy in 2013 and 2014, Mateen bought an an AR-15-style assault rifle, a Sig Sauer RMCX, on June 4 – just one week before the attack, the source says. He also had a Glock model 17 9mm and a Smith and Wesson .38 caliber revolver.
The first investigation by the FBI occurred after Mateen made concerning comments at work. As part of their investigation, the FBI interviewed witnesses, provided physical surveillance, and interviewed Mateen on two separate occasions, but the investigation was ultimately closed, according to a federal law enforcement source.
In 2014, the FBI opened a second investigation into Mateen for his ties to Florida-born Moner Mohammad Abu-Salha, the first known American suicide bomber – who blew himself up in Syria in 2014. Mateen was interviewed as part of the investigation, but there was not enough evidence at the time and the case was closed, the source says.
Over the previous month, Mateen used one of several Facebook accounts he operated to search the FBI and local police departments. He also looked up the San Bernadino shooters and a local gun range. On Sunday before the massacre, he posted about his allegiance to ISIS leader Abu Bakr al-Baghdadi, writing: "May Allah accept me."
According to police, Mateen opened fire just after 2 a.m. local time at Pulse, a popular gay club south of downtown Orlando. An off-duty officer working at the club initially responded to the shooter, "engaging in the gun battle with the suspect" before the suspect went back into the club, Orlando Police Chief John Mina said at a Sunday morning news conference.
The shooter held several people hostage for several hours. At 5 a.m., police moved in to rescue the hostages. More gunshots followed, and police also used explosives in an attempt to distract the suspect. Eventually, Mateen was shot and killed.
The gunman called 911 approximately 30 minutes after opening fire at Orlando's Pulse, a federal law enforcement official tells WHO. On that call, he announced he was attacking the club and claimed he was armed with explosives – a claim that authorities have not confirmed and are "still exploring," the official says.
"The 911 operator, using reverse-call technology, called back the gunman and it was then that he pledged allegiance to [ISIS leader] Abu Bakr al-Baghdadi," the federal official tells WHO. The gunman "also mentioned the Tsarnaev brothers," who orchestrated the Boston Marathon bombings of 2013, the official says.
ISIS also claimed responsibility for the attack.
"Agents in Afghanistan are looking at the shooter's connections there," an FBI source with knowledge of the investigation tells WHO. "He has family there. This is not to say his family is connected to the shootings. This is the normal course of an investigation like this. We ask questions."
"The Islamic State called for attacks on the West during Ramadan," the source continues. "In the wake of San Bernardino, we are very much mindful of how this can play out."