The suspected gunman behind the El Paso shooting that has left at least 20 people dead is believed to be a 21-year-old white man called Patrick Crusius.

Though Crusius was not named by law enforcement as the shooter, local media reported his name and published what it said were CCTV images of the suspect armed with a rifle.

The Texas city’s police chief said the assault on a Walmart store on Saturday, which left another 26 people wounded, was being investigated as a potential hate crime

Police officially identified a 21-year-old white male from Allen, Texas, a Dallas suburb some 650 miles east of El Paso.

The attack came just minutes after a far-right manifesto appeared online. Senior law enforcement officials told NBC News they were “reasonably confident” the document had been posted by the shooting suspect on online message board 8chan.

El Paso mass shooting: 20 killed at Walmart store Show all 39 1 /39 El Paso mass shooting: 20 killed at Walmart store El Paso mass shooting: 20 killed at Walmart store CCTV images of the gunman identified as Patrick Crusius The 21 year old, as he entered the Cielo Vista Walmart store in El Paso. The gunman was armed with an assault rifle and opened fire on shoppers at a packed Walmart store, killing 20. KTSM 9/AFP/Getty El Paso mass shooting: 20 killed at Walmart store Law enforcement agencies respond The Texas city’s police chief said the assault on a Walmart store on Saturday, which left another 26 people wounded, was being investigated as a potential hate crime. AFP/Getty El Paso mass shooting: 20 killed at Walmart store Shoppers exit with their hands up Shoppers exit with their hands up after a mass shooting at a Walmart in El Paso, Texas, U.S. August 3, 2019. REUTERS/Jorge Salgado NO RESALES. NO ARCHIVES. STRINGER Reuters El Paso mass shooting: 20 killed at Walmart store FBI released a picture of gunman Patrick Crusius The police officially identified the 21-year-old white male from Allen, Texas, a Dallas suburb some 650 miles east of El Paso. FBI/AP El Paso mass shooting: 20 killed at Walmart store A woman reacts after the mass shooting The attack came just minutes after a far-right manifesto appeared online. If authentic, it would make it the third mass shooting this year announced in advance on the website, which often features far-right and racist content. Reuters El Paso mass shooting: 20 killed at Walmart store Law enforcement responds to the active shooter The racist four-page document, titled “The Inconvenient Truth”, calls the Walmart attack “a response to the Hispanic invasion of Texas” and expresses support for the gunman who killed 51 people at two mosques in Christchurch, New Zealand earlier this year. AFP/Getty El Paso mass shooting: 20 killed at Walmart store A shopper hiding with an old lady behind the return and exchanges counter as the shooting began. Aaron Castaneda/Reuters El Paso mass shooting: 20 killed at Walmart store Ambulances in the car park near the scene It is ranked as the eighth-deadliest mass shooting in modern US history, after a 1984 shooting in San Ysidro, California, that claimed 21 lives. AP El Paso mass shooting: 20 killed at Walmart store Kendall Long (left) comforts Kianna Long (right) who was in the freezer section of Walmart. EPA El Paso mass shooting: 20 killed at Walmart store epa07755367 Police stand at attention after a shooting at a Walmart in El Paso, Texas, USA, 03 August 2019. According to reports, at least one person was killed and at least 18 people injured and transported to local hospitals. One suspect is in custody. EPA/IVAN PIERRE AGUIRRE IVAN PIERRE AGUIRRE EPA El Paso mass shooting: 20 killed at Walmart store Shoppers exit with their hands up. Reuters El Paso mass shooting: 20 killed at Walmart store Shopping carts sit next to a curb after the shooting. EPA El Paso mass shooting: 20 killed at Walmart store El Paso Fire Medical personnel arrive at the scene. AP El Paso mass shooting: 20 killed at Walmart store Walmart employees react after. AP El Paso mass shooting: 20 killed at Walmart store A woman runs to police near the scene. AP El Paso mass shooting: 20 killed at Walmart store Several law enforcement agencies respond. AP El Paso mass shooting: 20 killed at Walmart store People arrive at MacArthur Elementary looking for family and friends as the school is being used a re-unification centre. AP El Paso mass shooting: 20 killed at Walmart store Residents Erica Rios, 36, and Alma Rios, 61, cry outside a reunification centre. AFP/Getty El Paso mass shooting: 20 killed at Walmart store People gather in Juarez, Mexico, in a vigil for the Mexican nationals who were killed. AP El Paso mass shooting: 20 killed at Walmart store A child takes part in a vigil in Ciudad Juarez AFP/Getty El Paso mass shooting: 20 killed at Walmart store Francisco Castaneda joins mourners taking part in a vigil at El Paso High School. Reuters El Paso mass shooting: 20 killed at Walmart store Presidential candidate and former congressman Beto O'Rourke, right, meets with mass shooting survivor, Rosemary, at University Medical Centre Beto O'Rourke Facebook via AP El Paso mass shooting: 20 killed at Walmart store From left, Melody Stout, Hannah Payan, Aaliyah Alba, Sherie Gramlich and Laura Barrios comfort each other during a vigil for victims of the shooting. AP El Paso mass shooting: 20 killed at Walmart store A sign is posted near the scene Getty Images El Paso mass shooting: 20 killed at Walmart store Police cars parked below the Walmart sign block a road outside while investigating. EPA El Paso mass shooting: 20 killed at Walmart store Sherie Gramlich reacts during a vigil. AP El Paso mass shooting: 20 killed at Walmart store A man places flowers at the site Reuters El Paso mass shooting: 20 killed at Walmart store People take part in a rally against hate a day after a mass shooting at the Walmart store Reuters El Paso mass shooting: 20 killed at Walmart store Lupe Lopez holds a picture of a victim during a vigil for victims AP El Paso mass shooting: 20 killed at Walmart store People take part in a rally against hate a day after a mass shooting at a Walmart store, in El Paso, Texas, U.S. August 4, 2019. REUTERS/Jose Luis Gonzalez JOSE LUIS GONZALEZ REUTERS El Paso mass shooting: 20 killed at Walmart store Elsa Mendoza Marquez, a Mexican schoolteacher who was married and the mother of two adult children, was one of the victims Getty El Paso mass shooting: 20 killed at Walmart store People with the Mexican flag and the US flag take part in a rally against hate a day Reuters El Paso mass shooting: 20 killed at Walmart store People raise their arms in the air during a vigil for victims AP El Paso mass shooting: 20 killed at Walmart store People react and embrace each other Getty El Paso mass shooting: 20 killed at Walmart store Women light candles at a make shift memorial at the site of a mass shooting EPA El Paso mass shooting: 20 killed at Walmart store Adria Gonzalez (centre) who is being hailed as a hero for leading some Walmart customers to safety, speaks to the crowd AFP/Getty El Paso mass shooting: 20 killed at Walmart store People hold up their phones AFP/Getty Images El Paso mass shooting: 20 killed at Walmart store Beto O'Rourke speaks to the crowd AFP El Paso mass shooting: 20 killed at Walmart store People react during a prayer and candle vigil organized by the city, after a shooting left 20 people dead at the Cielo Vista Mall WalMart in El Paso, Texas, on August 4, 2019. - A shooting at a Walmart store in Texas left multiple people dead. At least one suspect was taken into custody after the shooting in the border city of El Paso, triggering fear and panic among weekend shoppers as well as widespread condemnation. It was the second fatal shooting in less than a week at a Walmart store in the US and comes after a mass shooting in California last weekend. (Photo by Mark RALSTON / AFP)MARK RALSTON/AFP/Getty Images MARK RALSTON AFP/Getty

If authentic, it would make it the third mass shooting this year announced in advance on the website, which often features far-right and racist content.

El Paso police chief Greg Allen said authorities were examining the manifesto, which indicated “there is a potential nexus to a hate crime”. Officials declined to elaborate and said the investigation was continuing.

The racist four-page document calls the Walmart attack “a response to the Hispanic invasion of Texas” and expresses support for the gunman who killed 51 people at two mosques in Christchurch, New Zealand, earlier this year.

He rails against automation, the “destruction of our environment” and immigration, and says his views were influenced by the so-called “Great Replacement”, a white supremacist conspiracy theory that claims people of European descent are being overwhelmed.

The 2,300-word diatribe expresses hatred of “race mixing” and suggests the US should be split up into different regions for different ethnicities.

His opinions on immigration, he writes, predate “Trump and his campaign for president”, though the author repeatedly uses talking points often wielded by the US president, including the claim “illegals” are “invading” the country, and that the mainstream media publishes “fake news”.

CNN reported the FBI has opened a domestic terror investigation into the shooting.

El Paso police say a 'white male in his 20s' is in custody

At least two Democratic presidential candidates, Pete Buttigieg and El Paso native Beto O’Rourke, drew connections to a resurgence in white nationalism and xenophobic politics in the US.

“America is under attack from homegrown white nationalist terrorism,” Mr Buttigieg said at a candidates forum in Las Vegas.

Mr O’Rourke partially blamed Donald Trump’s racist rhetoric, which “fundamentally changes the character of the country – and it leads to violence”.

On Twitter, Mr Trump branded the shooting “an act of cowardice”, adding, “I know that I stand with everyone in this country to condemn today’s hateful act. There are no reasons or excuses that will ever justify killing innocent people”.

The carnage ranked as the eighth-deadliest mass shooting in modern US history, after a 1984 shooting in San Ysidro, California, that claimed 21 lives.

It came just six days after the last major outbreak of US gun violence in a public place – a food festival in California where a teenager killed three people with an assault rifle and injured a dozen others before taking his own life in a hail of police gunfire.

(EPA)

Just hours after the El Paso attack, a gunman shot dead at least nine people outside a bar in Dayton, Ohio, early on Sunday morning.

“We are going to aggressively prosecute it both as capital murder but also as a hate crime, which is exactly what it appears to be,” Texas governor Greg Abbott told reporters of the Walmart attack, adding, “I don’t want to get ahead of the evidence”.

Refusing to call for tightening gun control measures, Mr Abbott said it was time to “focus more on memorials before we start the politics”.

In an emotional statement, El Paso county sheriff Richard Wiles railed against those “jumping in front of the cameras and offering prayers and condolences as things just keep getting worse”.

“It’s time to rise up and hold our representatives accountable at all levels. I want representatives who will stand up to racism. Who will stand up and support the diversity of our nation and our state,” he said.