(CNN) 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.

Athena Castilla, left, with her sister Andrea during the weekend of the Route 91 Harvest country music festival in Las Vegas. Andrea was fatally shot at the festival the night of Sunday, October 1. (Photo provided by Castilla family)

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?"

Photos: Mass shooting at Las Vegas music festival Debris is scattered on the ground Monday, October 2, at the site of a country music festival held this past weekend in Las Vegas. Dozens of people were killed and hundreds were injured Sunday when a gunman opened fire on the crowd. Police said the gunman fired from the Mandalay Bay hotel, several hundred feet southwest of the concert grounds. It is the deadliest mass shooting in modern US history. Hide Caption 1 of 30 Photos: Mass shooting at Las Vegas music festival Broken windows of the Mandalay Bay are seen early in Las Vegas on Monday. Police said the gunman fired on the crowd from the 32nd floor of the hotel. Hide Caption 2 of 30 Photos: Mass shooting at Las Vegas music festival People cross a street near the Las Vegas Strip just after sunrise on Monday. Thousands were attending the music festival, Route 91 Harvest, when the shooting started. Hide Caption 3 of 30 Photos: Mass shooting at Las Vegas music festival People embrace outside the Thomas & Mack Center after the shooting. Hide Caption 4 of 30 Photos: Mass shooting at Las Vegas music festival Police arrive at the Sands Corporation plane hangar where some people ran to safety after the shooting. Hide Caption 5 of 30 Photos: Mass shooting at Las Vegas music festival A woman cries while hiding inside the Sands Corporation plane hangar. Hide Caption 6 of 30 Photos: Mass shooting at Las Vegas music festival Concertgoers dive over a fence to take cover from gunfire on Sunday night. Hide Caption 7 of 30 Photos: Mass shooting at Las Vegas music festival Police take position outside the Mandalay Bay. Hide Caption 8 of 30 Photos: Mass shooting at Las Vegas music festival A man lays on top of a woman as others flee the festival grounds. The woman reportedly got up from the scene. Hide Caption 9 of 30 Photos: Mass shooting at Las Vegas music festival Hide Caption 10 of 30 Photos: Mass shooting at Las Vegas music festival People are seen on the ground after the gunman opened fire. Hide Caption 11 of 30 Photos: Mass shooting at Las Vegas music festival People run from the festival grounds. Hide Caption 12 of 30 Photos: Mass shooting at Las Vegas music festival A woman is moved outside the Las Vegas Tropicana resort. Multiple victims were being transported to hospitals in the aftermath of the shooting. Hide Caption 13 of 30 Photos: Mass shooting at Las Vegas music festival People are searched by police at the Tropicana. Hide Caption 14 of 30 Photos: Mass shooting at Las Vegas music festival An ambulance leaves the intersection of Las Vegas Boulevard and Tropicana Avenue. Hide Caption 15 of 30 Photos: Mass shooting at Las Vegas music festival A man in a wheelchair is evacuated from the festival after gunfire was heard. Hide Caption 16 of 30 Photos: Mass shooting at Las Vegas music festival Victims of the shooting are tended to in the street. Hide Caption 17 of 30 Photos: Mass shooting at Las Vegas music festival Concertgoers help an injured person at the scene. Hide Caption 18 of 30 Photos: Mass shooting at Las Vegas music festival People gather around a victim outside the festival grounds. Hide Caption 19 of 30 Photos: Mass shooting at Las Vegas music festival A couple huddles after shots rang out at the festival. Hide Caption 20 of 30 Photos: Mass shooting at Las Vegas music festival An injured woman is helped at the Tropicana. Hide Caption 21 of 30 Photos: Mass shooting at Las Vegas music festival Police and emergency responders gather at the intersection of Las Vegas Boulevard and Tropicana Avenue. Hide Caption 22 of 30 Photos: Mass shooting at Las Vegas music festival A police officer takes position behind a truck. Hide Caption 23 of 30 Photos: Mass shooting at Las Vegas music festival A crowd takes cover at the festival grounds. Hide Caption 24 of 30 Photos: Mass shooting at Las Vegas music festival Police officers advise people to take cover in the wake of the shooting. Hide Caption 25 of 30 Photos: Mass shooting at Las Vegas music festival People tend to a victim at the festival grounds. Hide Caption 26 of 30 Photos: Mass shooting at Las Vegas music festival Police stand at the scene of the shooting. Hide Caption 27 of 30 Photos: Mass shooting at Las Vegas music festival A woman sits on a curb at the scene of the shooting. Hide Caption 28 of 30 Photos: Mass shooting at Las Vegas music festival Police are deployed to the scene. Hide Caption 29 of 30 Photos: Mass shooting at Las Vegas music festival A man makes a phone call as people run from the festival grounds. Hide Caption 30 of 30

'A unique case'

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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

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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.

Photos: Remembering the victims of the Las Vegas massacre Matthew Helms visits a makeshift memorial Tuesday, October 3, for the victims of Sunday night's mass shooting in Las Vegas. Helms worked as a medic during the shooting, which was the deadliest in modern US history. Hide Caption 1 of 17 Photos: Remembering the victims of the Las Vegas massacre American flags stand at half-staff as the sun rises over Washington on October 3. Hide Caption 2 of 17 Photos: Remembering the victims of the Las Vegas massacre A man leaves flowers outside the US Embassy in Moscow on October 3. Hide Caption 3 of 17 Photos: Remembering the victims of the Las Vegas massacre Lindsay Cotterman rests her head on Shawna Pieruschka's shoulder as they attend a candlelight vigil at the University of Nevada, Las Vegas on Monday, October 2. Hide Caption 4 of 17 Photos: Remembering the victims of the Las Vegas massacre Mourners attend a vigil at the corner of Las Vegas Boulevard and Sahara Avenue on October 2. Hide Caption 5 of 17 Photos: Remembering the victims of the Las Vegas massacre The lights of Paris' Eiffel Tower were switched off October 2 to pay tribute to the victims of the Las Vegas attacks and the victims of a knife attack in Marseille, France. Hide Caption 6 of 17 Photos: Remembering the victims of the Las Vegas massacre A Kansas City Chiefs fan holds up a sign during the "Monday Night Football" game between the Chiefs and the Washington Redskins. Hide Caption 7 of 17 Photos: Remembering the victims of the Las Vegas massacre The lights of the Empire State Building are dimmed in New York on October 2. Hide Caption 8 of 17 Photos: Remembering the victims of the Las Vegas massacre Country music star Keith Urban performs "Bridge Over Troubled Water" during a candlelight vigil in Nashville, Tennessee, on October 2. Hide Caption 9 of 17 Photos: Remembering the victims of the Las Vegas massacre People gather for the Nashville vigil on October 2. Hide Caption 10 of 17 Photos: Remembering the victims of the Las Vegas massacre City Hall in Tel Aviv, Israel, is lit up like the American flag on October 2. Hide Caption 11 of 17 Photos: Remembering the victims of the Las Vegas massacre New York University students attend a vigil on October 2. Hide Caption 12 of 17 Photos: Remembering the victims of the Las Vegas massacre Rosa and Alan Duarte attend a vigil at Las Vegas' City Hall on October 2. Hide Caption 13 of 17 Photos: Remembering the victims of the Las Vegas massacre From left, first lady Melania Trump, US President Donald Trump, Vice President Mike Pence and Pence's wife, Karen, take part in a moment of silence at the White House on October 2. Hide Caption 14 of 17 Photos: Remembering the victims of the Las Vegas massacre President Trump makes a statement about the mass shooting on October 2. "In times such as these, I know we are searching for some kind of meaning in the chaos, some kind of light in the darkness," he said. "The answers do not come easy. But we can take solace knowing that even the darkest space can be brightened by a single light, and even the most terrible despair can be illuminated by a single ray of hope." Hide Caption 15 of 17 Photos: Remembering the victims of the Las Vegas massacre Former US Rep. Gabrielle Giffords -- accompanied by her husband, Mark Kelly -- turns to shake her fist at the Capitol during a news conference on October 2. Giffords, who was shot by a constituent in 2011, has been calling on Congress to do more to address gun violence. Hide Caption 16 of 17 Photos: Remembering the victims of the Las Vegas massacre A moment of silence is held at the Martin Luther King Jr. Memorial in Washington during an event honoring the 60th anniversary of King's essay, "Nonviolence and Racial Justice." Hide Caption 17 of 17

'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.

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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."