A 21-year-old Texan armed with an assault rifle and a murderous rage turned a packed Walmart in El Paso into a slaughterhouse on Saturday, killing at least 20 people in one of the deadliest mass shootings in US history, police said.

A single suspect — identified by CNN as Patrick Crusius of Allen, TX a suburb of Dallas — was in custody hours after the mid-morning shooting, that left as many as 26 more people hospitalized, a police spokesman said.

“Twenty innocent people from El Paso have lost their lives and more than two dozen more are injured,” Texas Gov. Greg Abbott said at an evening press conference where police confirmed the staggering death toll. “We as a state unite in support of the victims and their family members. We want to do all we can to assist them.”

The youngest patient is a four-month-old baby who suffered traumatic injuries, hospital officials told CBS News.

The oldest among the injured is 82, ABC News reported.

The Walmart is near the sprawling Cielo Vista Mall, a popular shopping destination among Mexican tourists who cross into the border city to shop, then return home, the station reported.

“We have ruled out multiple shooters,” El Paso Police Department Sgt. Robert Gomez told reporters; officials had earlier said that witnesses reported seeing a pair of gunmen.

Law enforcement agencies sped to the scene, including local cops, state troopers, and federal agents with AFT, Homeland Security and border patrol.

The suspect was apprehended “without incident,” and without officers discharging their weapons, Gomez said.

“We have no information about what the motive is.”

But investigators are reviewing a manifesto — in which the author writes, “This attack is in response to the Hispanic invasion of Texas— believed to have been written by Crusius and posted online just a half hour before the shooting, CNN reported.

By early Sunday, probers were “reasonably confident” that Crusius wrote and posted the vile document, NBC reported.

A city of 680,000, El Paso sits just across from Juarez, Mexico.

“This is just a tragedy that I’m having a hard time getting my arms around,” El Paso Mayor Dee Margo told CNN.

The gunman began opening fire at around 10 a.m. local time, beginning his rampage in the store’s parking lot, an eyewitness told ABC News Radio.

Dressed in khaki pants and a dark, v-neck t-shirt, he looked like he was dancing as he stalked his victims, shopper Vanessa Saenz told the station.

He cornered a small crowd of cowering victims, raised his rifle, and started firing, she said. He then walked with confidence into the store, “Like he was on a mission.”

Bystander video shows the young gunman next entering the Walmart — through the automotive section, one witness recounted — with rifle leveled.

In the video, he is wearing the kind of ear protectors and safety glasses common to firing ranges.

He “started shooting everyone, aisle by aisle, with rage,” said shopper Miguel Rodriguez, who’d been shopping for a toy for his 7-year-old son when he heard the gunshots and ducked to the ground, he told the Daily Beast.

Shoppers– many buying back-to-school supplies, cowered under tables as shots rang through the store, according to one witness video published by the local CBS afilliate.

“People were panicking and running, saying that there was a shooter,” Kianna Long said. “They were running close to the floor, people were dropping on the floor,” said Long, who escaped through the stock room and sheltered in a steel container with other customers in the store’s shipping area.

Britney, 19, told the news site that she was with her mother and little brother in the Walmart’s underwear aisle when the shots rang out. The three dropped to the ground, waited for the shots to stop, then ran from the store, holding hands.

“My mom is dead,” a young boy told Fox News, after seeing what he described as multiple dead bodies.

“We heard shots and saw smoke,” Victor Gamboa, 18, told Patch.com. “I saw a man on the floor full of blood,” said the teen, who works at the McDonalds inside the Walmart. “He appeared to be dead. It happened very quickly.”

Video posted to Twitter shows customers being evacuated with their hands over their heads, as an officer shouts, “Hands in the air!”

Shoppers were evacuated through the parking lot, past bloodied corpses.

“Ay, no!” one horrified videographer screams as he sees a corpse with a massive head injury.

Three hours after the first reports of gunfire — officials at that point had Crusius in custody but were still investigating later-discounted eyewitness reports of a second gunman — were still calling the scene unsafe.

“We don’t believe there is an imminent threat,” El Paso Police Sgt. Enrique Carillo told reporters outside the mall.

“There is no active shooter currently, but that is not to say we have rendered the scene safe.” A family reunification center was set up at a nearby elementary school.

The shooting comes just six days after a teen gunman opened fire — again with an assault-style weapon — on a crowd at a garlic festival in Northern California, killing three people before fatally shooting himself.

“Terrible shootings in El Paso,” President Trump tweeted after 4 p.m. from his golf course in Bedminster NJ. “Reports are very bad, many killed. Working with State and Local authorities, and Law Enforcement,” he said.

“Spoke to Governor to pledge total support of Federal Government. God be with you all!” Texas Gov. Greg Abott called the shooting “heinous and senseless.”

Democratic presidential candidate Beto O’Rourke, an El Paso native and former Texas congressman, said he was “incredibly saddened.” His campaign cancelled its weekend events in Nevada and California.

“I’ll tell you,” he told reporters during a candidate forum in Nevada.

“El Paso is the strongest place in the world. The community is going to stay together … Everyone’s resolved to make sure this doesn’t continue to happen in this country.”

Walmart released a statement saying, “We’re in shock over the tragic events at Cielo Vista Mall in El Paso.” The statement continued, “We’re praying for the victims, the community & our associates, as well as the first responders.

“We are working closely with law enforcement & will update as appropriate.”

With Post wires