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LAS VEGAS — Athena Castilla struggles to concentrate, the words slipping from her sentences as she tries to describe her sister, Andrea.

“She was just so silly all the time, happy, constantly taking videos and pictures and recording her life,” says Athena. “Always just full of life.”

Andrea Castilla was celebrating another year of life, her 28th birthday, with her boyfriend and her sister at the Route 91 Harvest country music festival. “It was a perfect weekend,” says Athena. She pauses and blinks, remembering.

First came the sound. Then, the panic.

“We started running and ducking. At the same time, everyone is running. It was chaos. Everyone was afraid. People were yelling, screaming, crying. Next thing you know, I hear her say, ‘Sis, duck! Stay down!'”

Athena turned and realized her sister had been shot. Stay with her, stay with her, Athena kept telling herself.

“It just felt like a rain of bullets, the scariest thing you could ever imagine. Not having any cover or anything to protect you. You’re just praying you’re going to make it out, praying not to get shot.”

Athena and her friends would carry her wounded sister out of the festival on a metal barrier, using it like a stretcher. At the hospital, Andrea Castilla would die from that fatal gunshot wound to the head.

“It doesn’t make sense at all,” says her father, Gus Castilla. “It’s why, why all this happened. That’s the big question. Why?”

‘A unique case’

Three weeks after the mass shooting in Las Vegas, investigators continue to ask the same question. Stephen Paddock, 64, left no apparent clues as to his motive, had no ties to political or extremist groups and left no note explaining why he would meticulously plan the worst mass shooting in modern US history.

“Usually within 24, 48 hours after an incident like this, we generally know what the motive is,” said Art Roderick, retired assistant director of the US Marshals Service and a CNN law enforcement analyst. “In my opinion, I think he doesn’t want us to know. He wants us to continue to ask these questions. This is a unique case. This individual is almost in a category by himself.”

Law enforcement is now left trying to piece together the gunman’s history, searching for strands in why a well-off retiree would turn to such violence.

Neighbors in Paddock’s senior living community recall him as reclusive, erecting a privacy fence that the housing association forced him to remove. The real estate agents who sold him his home say he paid the approximate $370,000 asking price in cash. Law enforcement sources are reviewing Paddock’s affinity for cruises that he and girlfriend Marilou Danley would take to European and Middle Eastern ports of call.

It was a comfortable lifestyle he managed to pay for with gambling.

Gambling and weapons

In a 2013 deposition that was a part of a civil lawsuit Paddock filed against the Cosmopolitan Hotel after he slipped and fell in the casino, Paddock testified he was the “biggest video poker player in the world.” He gambled $1 million in a single night, he testified, gambling in the overnight hours and sleeping during the day.

He also testified that he paid a yearly retainer fee to Nevada internist Steven Winkler, who prescribed him valium for “anxiousness.” Dr. Winkler did not return CNN’s repeated requests for comment.

Beyond gambling, Paddock’s other focus was weapons. A law enforcement source says Paddock had a history of gun purchases over more than 20 years, but he began accumulating more weapons in October 2016. The law enforcement source says starting that month, he bought 33 firearms, mainly rifles.

He purchased weapons in Nevada, Utah, Texas and California, raising no red flags at Dixie Gun Worx in St. George, Utah. “Every federal regulation that is in place, he passed. He passed all our background checks. He passed every red flag,” said Chris Michel, owner of Dixie Gun Worx.

‘I saw blood’

In news conference after news conference, Las Vegas law enforcement said Paddock planned meticulously. He rented rooms in a luxury condo tower called The Ogden, overlooking the Life is Beautiful open-air festival, just one week before the Route 91 festival.

He checked in to the Mandalay Bay Resort and Casino, bringing in two dozen guns in 10 suitcases. A hotel employee accompanied Paddock on two occasions with his luggage, assisting him with the service elevator.

And in the days before the shooting, Paddock gambled, according to the Las Vegas Metropolitan Police Department. Sheriff Joe Lombardo said the LVMPD had learned of 200 instances of the gunman being seen in Las Vegas, always alone.

Paddock prepared in the corner hotel suite, placing cameras to watch approaching officers. He drilled the stairwell door shut next to his suite.

It was that barricading of the stairwell door that triggered an alarm, prompting hotel security to dispatch security guard Jesus Campos to the 32nd floor.

Campos, speaking to Ellen Degeneres on her talk show, said while he was checking the door, the gunman noticed him. “I heard rapid fire and at first, I took cover,” Campos recounted. “I felt a burning sensation. I went to go lift my pant leg up and I saw blood.”

Forty seconds or less after shooting Campos, the LVMPD says, security cameras captured the first shots being fired at the innocent crowd below.

The gunman left no note, just calculations scribbled on a notepad measuring the distance from his suite to the people below. The LVMPD says it has discovered no extreme political beliefs or associations to extremist groups.

The Castilla family filed a lawsuit October 17, seeking to force the MGM/Mandalay Bay to make public what they knew about Paddock and what security measures they had or failed to have in place.

“The only thing we hope for is to prevent this from happening again,” said Athena Castilla, saying it would be what her sister would want. “We want to spread the message that (my sister) didn’t deserve what happened that night.”