ISIS may claim the attack (why wouldn't they?) but there seems to have be no evidence so far that they had anything to do with planning and executing it. It seems instead to be carried out entirely by a man disgusted by the sight of two men kissing, as his father has reported. For all the event will be linked to terrorism, it is important to remember that this was also an act of hate toward vulnerable populations of LGBT people, primarily people of color.
Yes, the Islamic State is probably claiming the attack. It would be rediculous for the group not to claim ownership after the attacker pledged allegiance to their leader. However, that does not mean they planned the attack or had anything to do with it ahead of time. Likely, the attacker was a lone wolf wanting to claim allegiance after the attack.
Yes, I believe that the Islamic State is claiming the attack because the Orlando shooter dialed 911 and pledged his allegiance to ISIS before committing the heinous massacre. Once again, the Islamic State proves that it can influence lone wolf killers in the United States and all over the world without having direct contact with these individuals.
While this tragic incident will surely be investigated for a long time to come, ISIS does not seem to be behind the attack. From what we know, this horrible outcome is a result of someone who had some serious mental issues. From his wife, to fellow co-workers he was one who was NOT religious and had a lot of anger. He also seemed to be someone who didn't fit in anywhere. He wanted to be a cop in the U.S., but that didn't work. He at some point wanted to have a marriage, but that didn't work. He seemed to be a failure at most arenas of his life and wanted someone to blame. So he decided to take his anger out on a large group of people who wouldn't be able to defend themselves.
I think he merely claimed "Isis" at the end so he could feel like he could be a part of some cause rather than admitting that he was a failure in life and wasn't accepted by anybody.