A routine summer Saturday in El Paso shattered by a massacre

In El Paso, the first sign that an ordinary Saturday would spiral into chaos confronted Vanessa Saenz in a parking lot.

At around 10 a.m., she was searching for an empty spot outside the Walmart -- just as she does every weekend when she shops with her family. The store was already teeming with back-to-school shopping. Suddenly, she said she heard "loud pops."

She assumed it was fireworks.

In fact, it was gunfire.

"I see this man — he looked like he was dancing, just hopping around," Saenz told ABC News. Then, to her right, she saw a woman walking her grocery cart to her car. "All of a sudden she just fell back. And then I thought, 'Those weren’t fireworks.'"

Her mother told her: "Just go go go. And I tried to drive off, but there were people running through the parking lot. Running for safety."

"I had to be very careful. There were kids running with the parents, and it was chaos at that point," she said. "He was walking toward the door. At one point he stopped, and he turned to his right, and just started — he pointed his rifle down and just started shooting, assuming at people who were there. He shot a couple of times, maybe two or three times."

By the time it was over, 20 people would be dead and another 26 injured.

The shooter, who police would later identify as a 21-year old white man, was dressed in cargo pants and wore ear protectors and a dark T-shirt, she said.

"He was very calm. He was just shooting randomly, at people that were nearby," Saenz said. "First thing i was thinking of was my mom's safety, my son's safety, myself. But I couldn’t stop thinking of people laying in the parking lot. As soon as I saw him walk into the store, I just — you know, you think of the people inside, you think of who is not going to see their loved ones at the end of the day."

For fellow El Paso resident, Robert Curado, terror met him just outside the store.

Curado and his mother hid between two vending machines.

"The individual tried to shoot at me, which he missed cause I kind of ducked down," he told the El Paso Times.

By 10: 39 a.m., the first 911 calls were coming in. Six excruciating minutes later, the police responded to the scene.

Meanwhile, confusion reigned at the nearby JC Penney in the adjacent Cielo Vista Mall.

In the middle of shopping, Victoria Balderrama heard an announcement they were closing the store down, she said. A few minutes later, she heard there was an active shooter at the Walmart, and possibly an active shooter at the mall.

"The SWAT team ended up coming into the JC Penny, and uh, they made us like run through the mall with our hands up. And we exited through the Dillards," Balderrama told ABC News. "There was lots of little kids and elderly people."

"It was so hot," Balderrama said. Temperatures in El Paso hovered over 100 degrees, according to weather reports.

Amid the heat and confusion, Balderrama said, there were "people running, screaming."

For others nearby, uncertainty stretched into the early afternoon, as they remained under lock-down.

“We’re all pretty nervous and scared,” Mistey Garcia, 33, an employee at a Hooters restaurant in the adjacent Cielo Vista Mall told ABC News in a telephone interview. “We’re kind of hoping and sending our prayers down to everyone at Walmart.”

Minutes before noon, everyone at Hooters remained at the restaurant, and no one had been briefed directly by law enforcement about whether the shooting was unfolding in or near Walmart, Garcia said. None of her coworkers had directly heard gunshots, but the wail of sirens continued.

Even after the imminent threat was over, the scene that greeted Jorge Salgado, a photojournalist at El Paso Inc, encapsulated the shock of people held hostage by uncertainty.

"I think everyone was just unsure," Salgado said. "Some people were crying, hugging. Some people were just — did not know what was going on. I spoke to one gentleman… he heard the gunshots....he said he tried to help some of the kids out of the mall ... he did his best to help people out."

For Salgado, who has been living in the area since 2002, the horror was jarring. El Paso, sits across the border from Juarez, Mexico, and the Walmart is a popular destination for Mexicans and locals alike to shop.

The Cielo Vista Mall, hosted "a lot of crossover with school just starting," he said, adding, a lot of the American and Mexican kids were "just trying to get ready for school."

ABC News' Chris Francescani, Desiree Adib, Sabina Ghebremedian, Tom Llamas and others contributed to this report..