Much of the motivation and thinking of the Las Vegas gunman remains entirely mysterious. But more and more is emerging about how Stephen Paddock prepared for his attack, the most deadly shooting in modern history.

Investigators have revealed what the killer was searching for in the days before the attack. And it offers an insight into the mind of the man who would go on to kill 58 people in his attack.

Paddock's search history shows him looking for precise details about how Las Vegas police and SWAT would break into a shooter's room, an anonymous investigator told the Wall Street Journal.

The discovery came just as it was reported that investigators had also found chilling details about Paddock's computer.

Investigators suspect gunman 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 told the Associated Press.

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. Investigators have examined his politics, finances, any possible radicalization and his social behavior — typical investigative avenues that have helped uncover the motive in past shootings — and there's still no clear motive.

Las Vegas shooting: Who is gunman Stephen Paddock?

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

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

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

Las Vegas shooting – in pictures Show all 15 1 /15 Las Vegas shooting – in pictures Las Vegas shooting – in pictures People scramble for shelter at the Route 91 Harvest country music festival after gun fire was heard Getty Las Vegas shooting – in pictures People carry a person at the Route 91 Harvest country music festival after shots were fired David Becker/Getty Las Vegas shooting – in pictures People run from the Route 91 Harvest country music festival after gun fire was heard David Becker/Getty Las Vegas shooting – in pictures A handout photo released via Twitter by Eiki Hrafnsson (@EirikurH) showing concertgoers running away from the scene (C) after shots range out at the Route 91 Harvest festival on Las Vegas Boulevard EPA/Eiki Hrafnsson Las Vegas shooting – in pictures People lie on the ground at the Route 91 Harvest country music festival after hearing gun fire Getty Las Vegas shooting – in pictures A man in a wheelchair is taken away from the Route 91 Harvest country music festival after hearing gun fire David Becker/Getty Las Vegas shooting – in pictures People stand on the street outside the Mandalay Bay hotel near the scene of the Route 91 Harvest festival on Las Vegas Boulevard EPA/Paul Buck Las Vegas shooting – in pictures FBI agents confer in front of the Tropicana hotel-casino after a mass shooting during a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Las Vegas police run by a banner on the fence at the Route 91 Harvest country music festival grounds after shots were fired David Becker/Getty Las Vegas shooting – in pictures An injured person is tended to in the intersection of Tropicana Ave. and Las Vegas Boulevard after a mass shooting at a country music festival Ethan Miller/Getty Las Vegas shooting – in pictures Metro Police officers pass by the front of the Tropicana hotel-casino after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures A woman sits on a curb at the scene of a shooting outside of a music festival along the Las Vegas Strip AP/John Locher Las Vegas shooting – in pictures A cowboy hat lays in the street after shots were fired near a country music festival in Las Vegas Getty Las Vegas shooting – in pictures Las Vegas Metro Police and medical workers stage in the intersection of Tropicana Avenue and Las Vegas Boulevard South after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Sheriff Joe Lombardo (2-R) speaking during a press briefing in the aftermath of the active shooter incident on Las Vegas Boulevard EPA

"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 $300,000 and $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 $1 million award from "justice assistance" funds to Nevada to help defray immediate costs of responding to the shooting.

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On Tuesday, Nevada Gov. Brian Sandoval and state Attorney General Adam Laxalt allocated $600,000 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 2,700 sworn officers, has an annual budget of $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 $6.6 billion.