By Christina Maxouris, Dakin Andone, Amir Vera and Nicole Chavez, CNN

(CNN) -- It took the first officer six minutes to arrive to an El Paso, Texas, Walmart on Saturday morning after reports of an active shooter.

By then, a massacre that would become one of the 10 deadliest in modern US history had already unfolded. A 21-year-old white supremacist is suspected of killing at least 20 people and injuring 26 others in the shooting -- one of at least three to devastate residents across the US in the past week.

Shocked shoppers slid under tables, others ran for their lives, one mother shielded her infant from the spray of bullets while another ran away with her 7-year-old daughter.

The suspect -- who sources identified to CNN as Patrick Crusius of Allen, Texas -- has been charged with capital murder and is being held without bond, El Paso Police Sgt. Robert Gomez said. He was arrested without incident Saturday after getting out of his vehicle and approaching police unarmed as they arrived at the Walmart. He has been cooperating with authorities, Gomez said.

As El Paso Police Chief Greg Allen walked into the shooting's aftermath, the scene was "horrific," he said.

"When I first got to this job," he said, "I never knew there was an odor to blood, but there is ... It will leave an impression that you'll never forget."

Suspect wrote a 'manifesto' police say

Authorities are now investigating a racist, anti-immigrant document they believe was posted online by the suspect. That document states it took less than a month to plan the shooting.

It was published on the online message board 8chan about 20 minutes before the shooting started. It lays out a dark vision of America overrun by Hispanic immigrants.

The 2,300-word "manifesto," as police called it, was attached to a post that read: "I'm probably going to die today."

The document is filled with white supremacist language and racist hatred aimed at immigrants and Latinos and blames immigrants and first-generation Americans for taking away jobs.

The writer cited a fear that an influential Hispanic population in Texas would make the state a "Democratic stronghold" and said "the Republican Party is also terrible" because the GOP is in his mind pro-corporation, which could lead to more immigration.

The writer said he held these beliefs before Donald Trump became President.

He could face the death penalty

Federal authorities are treating the shooting as a case of domestic terrorism, the US Attorney for the Western District of Texas said Sunday, as it seems to fit the statutory domestic terrorism definition. It "appears to be designed to intimidate a civilian population, to say the least," US Attorney John Bash said.

The Justice Department is also "seriously considering" bringing federal hate crime and federal firearm charges, which carry a possible death penalty, he said.

"We're going to do what we do to terrorists in this country, which is to deliver swift and certain justice," US Attorney John Bash said.

FBI orders scouring for more mass shooting threats

Following a week of deadly shootings in Texas, Ohio and California, FBI Director Chris Wray ordered the agency's offices across the country to conduct a new threat assessment in an effort to thwart future mass attacks, law enforcement sources told CNN.

A command group at the bureau's Washington headquarters will oversee the effort, the sources said.

The agency also said it's concerned that these and other attacks may inspire US-based domestic violent extremists to "engage in similar acts of violence."

"The FBI asks the American public to report to law enforcement any suspicious activity that is observed either in person or online," the FBI said in a Sunday statement.

The FBI already established a "fusion cell" this past spring to focus on white supremacists and hate crimes.

"Composed of subject matter experts from both the Criminal Investigative and Counterterrorism Divisions, the fusion cell offers program coordination from FBI Headquarters, helps ensure seamless information sharing across divisions, and augments investigative resources," the FBI said in their Sunday statement.

Among the victims was a mother who shielded her baby

Police are still in the process of notifying the families of victims in the El Paso shooting, Sergeant Robert Gomez said, adding authorities will not name any victims until all families have been notified. Police have said only that the victims are different ages and genders.

Some families have begun sharing their loved ones' stories.

Jordan and Andre Anchondo were shopping for school supplies in Walmart Saturday after dropping off their 5-year-old daughter to cheer practice.

The couple was killed in the massacre, but their 2-month-old son survived after his mom shielded him from the gunfire.

"The baby still had her blood on him. You watch these things and see these things and you never think this is going to happen to your family," Elizabeth Terry, Jordan Anchondo's aunt, told CNN.

Angie Englisbee, 86, was also killed.

Her son, Will Englisbee, told CNN his brother spoke with Angie Englisbee at 10:31 a.m. when she was in Walmart's check-out line. The first reports of an active shooter went out at 10:39 a.m. local time, the police chief said.

A 60-year-old Army veteran and bus driver, Arturo Benavides, was also killed, his niece told CNN.

"He was an absolutely caring and strong-willed man," Jacklin Luna said. "He was the person that would give any dime and shirt off his back, a meal and a home to anyone."

He loved telling stories of his Army days as a staff sergeant and life with his family.

"He deserves nothing less than the world to know everything he did and the love he had left to share," Luna said. "My nino didn't deserve this, neither did any of the beautiful people that were taken from us."

Leo Campos and Maribel Hernandez were also among those killed, according to CNN affiliate KFOX/KDBC.

They had dropped off their dog at the groomer before heading to Walmart, Hernandez's brother, Al Hernandez, told the affiliate. The family didn't know anything was wrong until the groomer called them and said the dog hadn't been picked up.

Seven Mexican nationals also lost their lives.

Secretary of Foreign Affairs Marcelo Ebrard identified them via Twitter as Sara Esther Regalado, Adolfo Cerros Hernández, Jorge Calvillo García, Elsa Mendoza de la Mora, Gloria Irma Márquez, María Eugenia Legarreta Rothe and Ivan Filberto Manzano.

Mexico explores legal action against US

On Sunday, Ebrard called the shooting an act of terrorism against Mexicans in the US and said in a press conference that the Mexican government will look into whether there is enough evidence to solicit the extradition of the gunman to face charges in Mexico.

El Paso County had an estimated population of 840,000 people as of July 2018, according to the US Census Bureau. An estimated 83% were Hispanic or Latino.

In a video posted on his Twitter, Ebrard said what happened in the Texas city was "unacceptable" and that "the first judicial actions" the government will take will be in accordance with international law.

"Mexico would like to express its utmost profound condemnation and rejection of this barbaric act where innocent Mexican men and women were killed," Ebrard said. "We are outraged. We do not support the culture of hate."

He also said Mexico President Andrés Manuel López Obrador instructed him to take swift legal action in order to first protect the families who were affected and then "so that Mexico can demand that the United States protect the Mexican community in the United States."

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