Kelley killed 26 people at a church in Sutherland Springs and then committed suicide.
Apple has said that it offered to help the FBI in the wake of the shootings once the agency said it was unable to unlock Kelley's encrypted smartphone. The fact that the phone was an Apple device was reported in some US newspapers.
Last year, Apple got involved in a major stoush with the FBI over a demand that it break into an iPhone which belonged to one of the attackers involved in a terrorist attack in San Bernardino, California, in December 2015. That stoush ended with the FBI backing down after it found a third party that was able to break into the iPhone in question.
Apple has a policy that material in an iCloud account can be given to law enforcement authorities if they have proper search warrants.
The newspaper said that Kelley had been found with an iPhone SE, and a second phone, an LG 328BG. Investigators are looking to obtain data from the iPhone and the shooter's iCloud account from 1 January 2016 onwards.