The motive of the shooter behind the deadliest mass shooting in U.S. history is unknown as of this writing.  What is known is that at least 50 people are dead and more than 400 were wounded in an attack on a country music concert directly below the Mandalay Bay hotel and casino in Las Vegas.

The gunman, Stephen Paddock, checked into the hotel on September 28 and managed to bring 10 rifles and thousands of rounds of ammunition into his room on the 32nd floor.  He reportedly set up two shooting platforms in the room, shot out the windows, and began to pour automatic weapon fire into the 22,000 concertgoers below.  Just as SWAT was about to break into his room, he apparently killed himself.  At least three long bursts of weapons fire were heard from his sniper perch, leading to the unprecedented carnage.

Who was he?

Las Vegas Review Journal:

"There is no reason we can imagine why Stephen would do something like this," his brother said. "All we can do is send our condolences to the people who died. Just no reason, no warning."

The younger Paddock repeatedly described his brother as "just a guy" with no serious health or money problems.

"As far as we knew, he had enough money to live the rest of his life in comfort," he said.

Paddock said he was on the phone all morning with Las Vegas police trying to figure out what happened.

"We're lost," he said. "We don't get it. This is like someone else did this."

Paddock said his brother lived a quiet life in retirement in Las Vegas, frequently playing slot machines and video poker on the Strip.

The last time the two brothers talked was a couple of weeks ago, when Stephen called to see how their mother was doing after Hurricane Irma swept through Orlando, he said.

Paddock said he was surprised to learn that his older brother had a cache of guns at his Mandalay Bay hotel room.

"He might have had a gun or two, but he didn't have a huge stock of guns," Paddock said.

The older Paddock bought a home in Mesquite in January 2015 for $369,000. His girlfriend, Marilou Danley, once considered a person of interest in the shooting investigation, lived with him in the 2,018-square-foot home, records show.

Court records show that Paddock does not have a criminal record in Las Vegas.

If Paddock snapped because of some mental illness, it didn't affect his ability to carefully plan the attack.  Each element of his planning was apparently meticulous, down to reportedly using cameras to record his attack and tell him when the police were about ready to break into his room.

So, a rational act?  Not necessarily.  He may have been severely delusional.  In such a mental state, he could have justified his actions while rationally planning the massacre.

I'm sure we'll find out more about Paddock over the next few days and weeks.  But at this point, there doesn't appear to be anything political that motivated him or any kind of obvious mental infirmity that led to his inexplicable and violent act.