Paddock purchased the items used in his attack during the year leading up to it, the Federal Bureau of Investigation said, and a large share of the ammunition and accessories he amassed appear to have been bought online.
One agent wrote that he believed "if there were any information related to a potential conspiracy, it would be found within" the locked phones.
A CNN review of public records shows that the email account linked to the sender is associated with a "Steve P". Paddock reportedly used guns equipped with the device, which allows semi-automatic weapons to fire nearly as quickly as automatic ones.
The documents also reveal that Paddock's girlfriend, Marilou Danley, had personally handled the stockpile of ammunition found in his home. Danley has denied knowing anything about Paddock's plans.
Her boyfriend, Stephen Paddock, shot himself dead after firing from a Las Vegas Strip casino into a concert crowd, killing 58 people and injuring hundreds.
The third phone, however, was unable to be unlocked.
Boiling lobsters alive ruled an act of cruelty in Switzerland
Photo taken early on December 23, 2014 shows cooked lobsters for sale in the auction house at the Sydney Fish Market in Sydney. But there's "clear-long-term motivational change [in these experiments] that's entirely consistent with the idea of pain".
Though Danley has not been charged, investigators note that "she has been identified thus far as the most likely person who aided or abetted Stephen Paddock based on her informing law enforcement that her fingerprints would likely be found on the ammunition used during the attack".
Law-enforcement officials believe the shooting was meticulously planned, and that after Paddock checked into the Mandalay Bay three days prior, to his room. In fact, Paddock took a number of steps aimed at tripping up investigators, and the Federal Bureau of Investigation team trying to get to the bottom of the shooting wrote that the 64-year-old exhibited "a level of sophistication which is commonly found in mass casualty events".
In her statement after the shooting, Danley said she had assumed Paddock was breaking up with her, continuing that it "never occurred to me in any way whatsoever that he was planning violence against anyone".
The court records also show that investigators sought to preserve the content of social media accounts on Instagram and Facebook linked to Paddock and Danely.
She notably deleted social media profiles in the hours after the attack. "Conversely, if the Target Account was not controlled by Stephen Paddock, investigators need to determine who was communicating with him about weapons that were used in the attack", according to a warrant.
The message read in part: "for a thrill try out bumpfire ar's with 100 round magazine", an apparent reference to police say that Paddock affixed to a number of the firearms found in his room.