A gunman opened fire at a Walmart in El Paso, Texas, Saturday morning, causing "multiple fatalities," city police and officials said.

Texas Lt. Gov. Dan Patrick told Fox News that there were between 15 and 20 "casualties" of the shooting, but did not give a number of fatalities. 

Police responded to reports of an active shooting in the area of the Cielo Vista Mall shortly before 11 a.m. and asked people to stay clear of the active scene. El Paso Police Sgt. Enrique Carrillo told reporters shortly before 1 p.m. that one person was in custody and there was no longer an "imminent threat" to the area.

Ryan Mielke, a spokesman for University Medical Center of El Paso, told Fox News that 13 patients -- 1 of whom died at the hospital -- were being treated at the facility's Level 1 trauma center. Mielke told the Associated Press that says two children, ages 2 and 9, were stabilized at the medical center before being transferred to the neighboring El Paso Children's Hospital.

Del Sol Medical Center told Fox News it has received 11 patients, 9 of whom were critical but stable condition and 2 who were in stable condition. The patients' ages ranged from 35 to 82, the hospital said. El Paso police tweeted that blood donations were "needed urgently" to aid the injured.