Officials are asking Arizonans to help victims of the mass shooting in El Paso, Texas by donating blood.

At least 20 people were killed and 26 wounded in the Saturday morning attack by a 20-year-old gunman.

Donation centers in Glendale, Goodyear, Mesa and Phoenix will be open to accept whole-blood and red-blood-cell donations Monday from 10:30 a.m. to 5:30 p.m.

Brad Daughtry, a spokesman for blood-donation non-profit Vitalint, explained that donors are urgently needed to replenish the blood that's already been used to help victims.

"Events like these are always a sad reminder that it's the blood already on the shelves that's used (for) victims to something like what happened in El Paso," he said.

UPDATE: These are the victims of the El Paso Walmart shooting

"It's important that people donate year-round so the blood is there when the victims need it," Daughtry added.

Donation centers in the Phoenix area have been busy as residents learned of the shooting, but there's still room for more donors, he said.

Vitalant's El Paso center sent 367 units of blood to hospitals treating the shooting victims, and additional blood collected in Arizona will be sent to hospitals in the border city, Daughtry said.

He added that donations of all blood types are welcome, but Type O is in especially short supply.

Where to donate blood

Phoenix: 5757 N. Black Canyon Highway.

Glendale: 18583 N. 59th Ave.

Mesa: 1337 S. Gilbert Road, #101.

Goodyear: 14270 W. Indian School Road, C-8.

Walk-ins are welcome, but appointments take priority. Make an appointment at bloodhero.com or 877-25-VITAL (877-258-4825).