While investigators are no closer to establishing what drove Vegas sniper Stephen Paddock to carry out his sick massacre, they have uncovered his alarming search history.

Authorities investigating the shooting that killed 58 concertgoers and injured more than 500 others have found internet search techniques used by local police for breaching rooms in standoff situations, a law-enforcement official reportedly told the Wall Street Journal.

The official reportedly said Paddock searched for information about how Las Vegas Metropolitan Police Department or SWAT teams force their way into rooms or buildings.

Las Vegas shooter's arsenal of weapons in the Mandalay Bay Hotel. Photo / AP
Las Vegas shooter's arsenal of weapons in the Mandalay Bay Hotel. Photo / AP

Investigators also discovered a laptop belonging to Paddock is missing its hard drive, making it more difficult to establish the motive in America's deadliest mass shooting.

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The internet query and the missing hard drive are two more pieces of the puzzle authorities are yet to solve. However, ABC News reported Paddock is believed to have removed the hard drive before fatally shooting himself.

Investigators suspect gunman Stephen Paddock removed the hard drive from the laptop after opened fire from his high-rise suite at the Mandalay Bay hotel casino into a crowd at a country music concert, killing 58 people and wounded hundreds of others, an official said.

Investigators have been combing through every aspect of Paddock's life - from family, friends and associates to his travel patterns, health and finances.

The hard drive hasn't been found, according to the official, and the absence of digital clues adds yet another puzzling aspect to the investigation as authorities try to figure out what might've led the 64-year-old high-stakes gambler to commit the mass killing.

Leaked photographs from inside the hotel room used by Las Vegas shooter Stephen Paddock in the Mandalay Bay Hotel. Photo / AP
Leaked photographs from inside the hotel room used by Las Vegas shooter Stephen Paddock in the Mandalay Bay Hotel. Photo / AP

They even examined the gunman's brain to determine whether he was suffering from dementia.

Microscopic tissue examination can reveal otherwise hidden conditions such as dementia and chronic traumatic encephalopathy or CTE.

Paddock's brain has so far shown no obvious sign of tumour, injury or abnormality during a more routine autopsy conducted last week by Vegas coroners, according to officials - who have yet to release toxicology results or an official cause and manner of death.

Bruce Paddock, the brother of Las Vegas shooter Stephen Paddock, was arrested in Los Angeles on suspicion of possessing child pornography.
Bruce Paddock, the brother of Las Vegas shooter Stephen Paddock, was arrested in Los Angeles on suspicion of possessing child pornography.
Flowers, candles and other items surround the famous Las Vegas sign at a makeshift memorial for victims of a mass shooting. Photo / AP
Flowers, candles and other items surround the famous Las Vegas sign at a makeshift memorial for victims of a mass shooting. Photo / AP

MASSACRE COSTS

On Wednesday, authorities also estimated that the costs of police, fire and other emergency services related to the massacre were projected at about US$4 million and climbing.

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The disclosure came amid pledges by the federal government to provide US$1 million, and from the state of Nevada to pay US$600,000 to defray costs stemming from the October 1 shooting.

Las Vegas police Officer Laura Meltzer provided what she called a preliminary figure of US$3.5 million for department costs associated with the worst mass shooting in modern US history.

"We still have a lot of resources devoted to the investigation," Meltzer said.

"We still have people working overtime and around the clock on it." The amount spent on overtime to date amounts to more than half a percent of the annual Las Vegas police budget.

Clark County administrators on Tuesday estimated fire department, coroner and social services costs at between US$300,000 and US$500,000, county spokesman Erik Pappa said.

"Over the long term, we expect it will be in the millions," he said. Pappa noted that a facility dubbed the "The Vegas Strong Resiliency Center" opened Monday to serve as a one-stop resource for references and referrals for residents, visitors and responders affected by the shooting. He said it is expected to remain open for several years.

Paddock, a former real estate investor with homes in Reno and Mesquite, Nevada, fired assault-style weapons out two 32nd-floor windows at the Mandalay Bay resort into a crowd of 22,000 people at an open-air country music concert across the street. Police and the FBI have said they believe he acted alone. The U.S. Justice Department last week announced a $US1 million ($A1.2 million) award from "justice assistance" funds to Nevada to help defray immediate costs of responding to the shooting.

On Tuesday, Nevada Gov. Brian Sandoval and state Attorney General Adam Laxalt allocated $US600,000 ($A778,000) from funds that Laxalt said were reaped from a deceptive trade practices lawsuit settlement in July. Laxalt called the money non-taxpayer funds.

Sandoval called the money a first step in providing "any additional resources necessary" to help the investigation, and said the police department should receive the state money within 15 days.

The Las Vegas Metropolitan Police Department, with almost 2700 sworn officers, has an annual budget of US$552 million. It covers most of Clark County, with a population of about 2 million people and more than 40 million visitors a year. The annual county budget is about US$6.6 billion.