The gunman who killed seven people and injured more than 20 in a shooting rampage around Odessa, Texas, purchased his weapon from a private seller, a transaction that does not require a background check, law enforcement officials told NBC News.

Investigators are now looking into who sold the weapon to 36-year-old Seth Ator, who failed a gun background check in 2014 because, law enforcement officials told NBC News, he had a disqualifying mental health issue.

If the person who sold the gun to Ator knew of his mental health issues, that seller could face criminal charges.

The House passed a bill in February that would require background checks for nearly all private sales, except those involving family members. But the Senate won't act on it until President Donald Trump makes his position clear.

A senior law enforcement official briefed on the case and a spokesperson for the Texas Department of Public Safety said the shooter had a rifle that may have been a .223 AR-15 style weapon.

Officials were also trying to determine whether the suspect had other weapons on him during the shooting or at his home.

The city of Odessa, late Monday afternoon, named all six of the adults killed: Joe Griffith, 40; Mary Granados, 29; Edwin Peregrino, 25; Rodolfo Julio Arco, 57; Kameron Karltess Brown, 30; and Raul Garcia, 35.

In addition to those killed, 25 others were injured in the rampage, including a 17-month-old girl.

Ator had been fired by his employer, Journey Oil Field Services, on Saturday, right before he went on his rampage in the neighboring cities of Odessa and Midland.

He was killed in a shootout with police.