Police have laid murder charges and are “seriously considering” bringing hate crimes charges in the case against the suspected gunman at the centre of a shooting rampage in El Paso, Texas.

Texas man Patrick Crusius, 21, was arrested after at least  20 people were shot dead and dozens wounded at a shopping centre in the Texas border town of El Paso.

The attack is being investigated as a possible hate crime after it emerged the killer posted a racist manifesto online praising the Christchurch massacre and white supremacy.

US Attorney for the Western District of Texas John F Bash told The Washington Post federal authorities were “seriously considering” laying hate crime charges, which could carry the death penalty.

“We are treating it as a domestic terrorism case and we’re going to do what we do to terrorists in this country, which is deliver swift and certain justice,” Mr Bash told the Post.

Local prosecutors also say they plan to seek a death sentence in the case.

Police responded Sunday morning (Australian time) to reports of an active shooter at the Cielo Vista Mall on the east side of the city, and locked down a five kilometre perimeter around the complex.

Alleged gunman Patrick Crusius. Photo: Facebook

One male suspect was taken into custody, with police saying they do not believe others were involved. CNN identified the suspect as 21-year-old Patrick Crusius of Allen, Texas,

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

Media reports said the shooting was concentrated in the centre’s Walmart discount department store.

The shooting comes just days after two employees were fatally shot at a Walmart store in Southaven, Mississippi, and three people were shot and killed at the Gilroy Garlic Festival in California.

Police issued an urgent plea for blood donations in El Paso to assist the medical effort.

Blood needed urgently. Multiple injured transported to various hospitals. Blood donation centers Vitalent Blood Services at 424 s Mesa Hills and 133 N Zaragoza

— EL PASO POLICE DEPT (@EPPOLICE) August 3, 2019

The latest US gun slaughter brought an immediate response from Prime Minister Scott Morrison, who tweeted his sympathy for victims of the “terrible and evil attack”.

Mr Morrison’s condolences were taken a step further by former deputy prime minister Tim Fischer, who called on Americans to stand up to the gun lobby and urged Australians to think twice before visiting the US.

Mr Fischer called on journalists to bail up visiting US Secretary of State Mike Pompeo and ask why tourists should risk their lives in travelling to the US.

“In the meantime Australia’s DFAT should upgrade the smart traveller site to reflect the dangerous situation in the USA re guns,” Mr Fischer said.

So terribly saddened by the senseless violence in El Paso that has taken so many innocent lives. Our thoughts and prayers are with the families and communities impacted as well as the first responders. A terrible and evil attack.

— Scott Morrison (@ScoMo30) August 4, 2019

“This was a massacre,” US Congresswoman Veronica Escobar, who represents the area, told CNN, adding that the death toll was  “shocking.”

“This was a massacre,” she said.

President Donald Trump took to Twitter to respond to the Texas shooting, describing it as “very bad”.

Terrible shootings in ElPaso, Texas. Reports are very bad, many killed. Working with State and Local authorities, and Law Enforcement. Spoke to Governor to pledge total support of Federal Government. God be with you all!

— Donald J. Trump (@realDonaldTrump) August 3, 2019

El Paso, which has about 680,000 residents, is in West Texas and sits across the border from Juarez, Mexico.

Texas state Governor Greg Abbott called the attack “a heinous and senseless act of violence”.

“We have deployed troopers, special agents, Texas Rangers, tactical teams, and aircraft to the scene in a support role,” Mr Abbott said.

https://twitter.com/emilnivantha/status/1157724962230161408

Presidential candidate and El Paso native Beto O’Rourke appeared shaken as he joined a candidate forum in Las Vegas on Saturday shortly after news of the shooting in his hometown was reported.

Mr O’Rourke said he had called his wife before taking the stage and said the shooting shatters “any illusion that we have that progress is inevitable” on tackling gun violence.

The Democrat said he’d heard early reports that the shooter might have had a military-style weapon, saying we need to “keep that (expletive) on the battlefield and do not bring it into our communities.

-with AAP