But Lombardo also said police didn't know Campos was hit until responding officers arrived on-scene later.
Paddock, who had installed three cameras to monitor the approach to his suite, opened fire through the door, spraying 200 shots down the hall and wounding the guard, who alerted other security officials, Lombardo said.
Sheriff Joseph Lombardo of the Las Vegas Metropolitan Police said Paddock shot and wounded Jesus Campos, a security guard at the Mandalay Bay Resort and Casino, shortly before 10 a.m. on October 1.
"It's because this individual purposely hid his actions leading up to this event, and it is hard for us to find the answers to those actions", Lombardo said.
The gunman's hotel room was on the 32nd floor of the Mandalay Bay hotel with a ideal view of the concert grounds where more than 20,000 people had gathered some 400 yards away for the Route 91 Harvest Musical Festival before Paddock launched his attack.
"I'm frustrated", Joseph Lombardo, the Las Vegas sheriff, said at a news briefing Monday night.
The body of a 28-year-old military veteran from California and Colorado who was killed in the worst mass shooting in modern American history is on its way home from Las Vegas.
And as concertgoers tried to run as shots rang out, they left behind almost everything.
Those who were in two areas of the concert grounds were being allowed to retrieve their things in groups, with authorities expanding the offer Monday to include people who were seated west of the stage. The bleacher seating to the east of the stage is also included, The AP reported.
Steinbeck says 99 people sought to recover their belongings. Police have asked people who lost their cellphones to text their full name to their phones. So I would be comfortable in saying, which I believe, dependent on the splash he made during the shooting would have enabled the first responders to be directing their attentions to other locations which would enable Mr. Paddock to just leave the hotel.
He says "extensive forensic analysis" is continuing.
Investigators have interviewed the suspect's brother, Eric Paddock, and other family members, according to Lombardo.
Bitsko said the level of preparation and work inside, including setting up wires for the cameras, suggested that it took Paddock "days to finish" getting it all ready.
Eric Paddock would not talk to The Associated Press by phone and declined by text message.
After Jesus Campos, a hotel security guard, took fire on the 32nd floor - Paddock blasted rounds through his door - officers headed up the stairs and found the barricaded stairwell door.
But so far, authorities looking to see if anyone may have aided Paddock found no evidence of the gunman having associates in Las Vegas.
The Las Vegas Review-Journal reports (http://bit.ly/2yzrWEz) that Eric Paddock arrived in town late Saturday for hours of interviews with Federal Bureau of Investigation agents, a police detective, a profiler and a psychologist.
Country star Jason Aldean has returned to Las Vegas a week after a mass shooting during his performance at a music festival left 58 people dead and hundreds more hurt.
Mr Newton described the "eerie" feeling of storming into Paddock's hotel room amid the chaos of the attack. She wrote in the caption that seeing the strength of victims helped the couple "try to begin the healing process".