Five days after the deadliest mass shooting in recent US history, authorities acknowledged Friday they have not uncovered a "clear motive" for the gunman.

"We still do not have a clear motive or reason why," Undersheriff Kevin McMahill of the Las Vegas Metropolitan Police Department told reporters.

"At this time I'll tell you we have no credible information to report to you as to motivation."

Stephen Paddock killed 58 people and left hundreds injured in Las Vegas after raining bullets on a crowd at an open-air country music concert while perched in a 32nd floor hotel room.