One thing we do know is there was some planning the guy made. Its not like he just decided on that single day to do the shootings because he bought the AR about a week before. So there was some planning. And depending on how many scouting missions LE can determine may give them a hint how far back he was beginning to plan the attack.
I will take him at his word for his main motivation he indicated on the 911 call.
"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.
"
Not sure what ties FBI is referring to here
"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"
"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."
"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 PEOPLE. The gunman "also mentioned the Tsarnaev brothers," who orchestrated the Boston Marathon bombings of 2013, the official says. "
http://www.people.com/article/omar-mateen-disney-world-scouted-attacks