Harper told reporters that federal agents were focusing their interviews related to the gunman around Dallas and San Antonio, making a plea for anyone with pictures or video from the shooting scene or the Dallas area to upload them to the FBI website, according to AP.
Harper stressed that the gunman apparently had no contacts in El Paso, AP said.
In what is now being considered an act of domestic terrorism in the US, over 20 people were killed and 26 more injured after a gunman - identified as 21-year-old white male Patrick Wood Crusius - opened fire in a Walmart in the Cielo Vista Mall in the city of El Paso, Texas, on Saturday evening.
The suspect was reportedly from a Dallas suburb over 600 miles from where Saturday's terror attack unfolded at a Walmart.
The United States was shaken on Saturday by two separate domestic terror attacks. Hours after the gun massacre in El Paso, another young white male gunman attacked people in the city of Dayton, Ohio, killing nine and injuring at least 26 people. The shooter's sister was among those killed in the Ohio gun massacre.
Trump ordered flags to be flown at half-staff to acknowledge the victims of terror attack and said that he would make a statement at 10 a.m. in Washington DC on Monday.