"We are treating [the El Paso shooting] as a domestic terrorist case, there's a statutory definition of domestic terrorism .... this meets it. It appears to be designed to intimidate a civilian population," US Attorney for Western District of Texas John Bash said at a press conference, Sunday.

He added that "we're going to do what we do to terrorists in this country, which is to deliver swift and certain justice."

The shooter, earlier identified in the media as 21-year-old Patrick Crusius, was said to be cooperating with the investigation.

The suspect is believed to have bought the firearm used during the rampage legally, El Paso Police Chief Greg Allen said.

Prosecutors will be seeking the death penalty for the suspect.

The El Paso massacre took place early on Saturday when the suspect, armed with an AK-style rifle, stormed a local Walmart shooting people in the parking lot and inside the store. At least 20 people were killed and 26 more injured.

A "manifesto" was posted online before the rampage, apparently detailing the shooter's extreme-right views and branding the attack a response to the "Hispanic invasion" of Texas. The document is yet to be directly linked to the shooter, police said on Sunday.

Local media reported that blood donation centers were"overwhelmed" after hundreds answered the calls from police and city officials for people to donate blood for the victims.

Residents and reporters at the scene shared videos on social media of people standing in long lines outside and inside the facilities. Some stood for hours, waiting for their turn to help the victims.

"I went to go donate blood but the facility didn't have room for any more cars and the streets were flooded with people trying to get in," one person wrote.

"My mother and I went to donate blood and there is a line of hundreds of people around the block. All standing in 101 degree sun," another Twitter user said. "People can be wonderful."

Sandra Ramirez, an anchor at the local KTSM 9 News channel, said that volunteers were bringing pizza, water and energy drinks to people enduring the heat to stand in lines in front of the blood donation centers.

Transport service Lyft, meanwhile, had offered free rides to people wishing to donate blood in El Paso.

The inflow of donors helped nonprofit Vitalant to receive more than 240 units of blood for the victims in just one day, the organization said.

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India has test-fired its state-of-the-art Quick Reaction Surface-to-Air Missile (QRSAM). The all-weather and all-terrain missile mounted on a truck successfully hit its targets during the tests conducted on Sunday at a range in the eastern Indian state of Odisha. The Indian defense minister has shared photos and a video of the missile launch on his Twitter account.

The new air defense system boasts an operational range of 25-30 kilometers. It is also equipped with anti-jamming devices allowing it to penetrate the enemy's defenses. The missile already underwent a series of trials in 2017 and in February 2019. On Sunday, two missiles were used to hit the targets flying at different altitudes under different conditions, Indian media reported citing military sources, who hailed the system's high maneuverability.