The gunman who killed 58 people and wounded more than 500 in Las Vegas last October may have been planning how to modify guns to make them fire hundreds of rounds a minute months earlier.
According to media reports on unsealed search warrants, Stephen Paddock sent emails discussing buying bump stocks months in advance of the deadliest mass shooting in US history.
Stocks believed to be used in the massacre were found in the 32nd-floor hotel room from where Paddock fired on a crowd gathered for the finale of a country music festival held on the Las Vegas Strip.
Details suggesting the attack may have been pre-planned are said to be part of more than 300 pages of search warrants unsealed by a federal judge in Nevada on Friday, according to the Los Angeles Times, one of several publications that sought release of the documents.
They do not provide a motive for the killings, the paper reported.
Paddock shot and killed himself before police arrived at his hotel room the night of the shootings.
The documents also showed that Paddock's girlfriend, Marilou Danley, told investigators before they searched a house the two shared that they might find her fingerprints on ammunition "because she occasionally participated in loading magazines", the Las Vegas Review-Journal reported.
In the days after the shootings, authorities called Danley a "person of interest". Her attorney has said she had no inkling of Paddock's plans.
Documents posted online by the Review-Journal showed FBI agents sought information from Microsoft and Facebook regarding the accounts held by Paddock and Danley.