It is a fact that ISIS claimed responsibility.
That is all I have said up to now.

CNN this morning,
"The attack fits a well-known pattern of ISIS recruitment and incitement: encouraging sympathizers via a sophisticated social media campaign to join the fight in Syria or, if they cannot do that, to carry out terror attacks on their own at home, U.S. officials said Tuesday.

Simpson appears to have had online ties to a British ISIS recruit thought to be in Syria and an American jihadi operating in Somalia, according to a CNN analysis of tweets exchanged in the lead up to Sunday's attack.

U.S. investigators suspect Junaid Hussain, a British hacker now believed to be with ISIS, might have played a key role in inspiring the Texan gunmen, a U.S. official told CNN.

And a CNN investigation has established that Simpson was in communication via Twitter a week before the attack with a man believed to Mohamed Abdullahi Hassan, an American who traveled to Somalia in 2008 to join the al Qaeda-affiliated terrorist group Al-Shabaab.

Hassan, believed to be tweeting under the name of Mujahid Miski, also put out a call one week before the event to attack the drawing contest for pictures of the Prophet Mohammed in Garland, Texas."