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.

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.

17 PHOTOS Mass shooting in Odessa, Texas See Gallery Mass shooting in Odessa, Texas Texas Gov. Greg Abbott gestures during a news conference concerning Saturday's shooting Sunday, Sept. 1, 2019, in Odessa, Texas. From left are Christopher Combs, FBI Special Agent in Charge, San Antonio, Odessa Police Chief Michael Gerke and Abbott. (AP Photo/Sue Ogrocki) Odessa Police Chief Michael Gerke announces that he does not want to speak the name of the shooter from Saturday's shooting during a news conference Sunday, Sept. 1, 2019, in Odessa, Texas. (AP Photo/Sue Ogrocki) Odessa Police Chief Michael Gerke, center, answers a question concerning Saturday's shooting during a news conference Sunday, Sept. 1, 2019, in Odessa, Texas. From left are Christopher Combs, FBI Special Agent in Charge, San Antonio, Gerke, and Texas Governor Greg Abbott. (AP Photo/Sue Ogrocki) Texas Governor Greg Abbott, right, pats Odessa Police Chief Michael Gerke, left, on the shoulder during a news conference about Saturday's shooting, Sunday, Sept. 1, 2019, in Odessa, Texas. (AP Photo/Sue Ogrocki) Law enforcement officials process a scene involved in Saturday's shooting, Sunday, Sept. 1, 2019, in Odessa, Texas. The death toll in the West Texas shooting rampage increased Sunday as authorities investigated why a man stopped by state troopers for failing to signal a left turn opened fire on them and fled, shooting more than a dozen people as he drove before being killed by officers outside a movie theater. (AP Photo/Sue Ogrocki) A city of Odessa police car, left, and a U.S. mail vehicle, right, which were involved in Saturday's shooting, are pictured outside the Cinergy entertainment center, Sunday, Sept. 1, 2019, in Odessa, Texas. The death toll in the West Texas shooting rampage increased Sunday as authorities investigated why a man stopped by state troopers for failing to signal a left turn opened fire on them and fled, shooting over a dozen people as he drove before being killed by officers outside a movie theater. (AP Photo/Sue Ogrocki) A U.S. Mail vehicle, right, which was involved in Saturday's shooting, is pictured outside the Cinergy entertainment center Sunday, Sept. 1, 2019, in Odessa, Texas. The death toll in the West Texas shooting rampage increased Sunday as authorities investigated why a man stopped by state troopers for failing to signal a left turn opened fire on them and fled, shooting over a dozen people as he drove before being killed by officers outside a movie theater. A police vehicle is partially blocked at left. (AP Photo/Sue Ogrocki) Agentes acordonan un tramo de acera en una calle en Odessa, Texas, el sábado 31 de agosto de 2019, después de que un hombre armado secuestrara un vehículo del servicio postal y baleara a más de 20 personas, según las autoridades. (Mark Rogers/Odessa American vía AP) Un hombre reza ante la unidad de urgencias del Medical Center Hospital en Odessa, Texas, el sábado 31 de agosto de 2019 tras un tiroteo en la zona de Odessa y Midland, en el oes de Texas. (Mark Rogers/Odessa American via AP) Odessa Police Chief Michael Gerke talks to the media in the Odessa Police Department in Odessa, Texas, Saturday, Aug. 31, 2019, after a man fired at random in the area of Odessa and Midland. Several people were dead after a gunman who hijacked a postal service vehicle in West Texas shot more than 20 people, authorities said Saturday. The gunman was killed and a few law enforcement officers were among the injured. (Mark Rogers/Odessa American via AP) ODESSA, TX - SEPTEMBER 1: A damaged police vehicle and U.S. Postal Service van blocked off with tape nearby to where a gunman was shot and killed at Cinergy Odessa movie theater following a deadly shooting spree on September 1, 2019 in Odessa, Texas. Seven people had been killed, in addition to the gunman and at least 21 others were wounded, including three law enforcement officers after a gunman went on a rampage. The man who has not been identified fled from state troopers who had tried to pull him over. The gunman then hijacked a United States postal van and indiscriminately fired from a rifle at people before the authorities shot and killed him outside a movie theater in Odessa. (Photo by Cengiz Yar/Getty Images) ODESSA, TX - SEPTEMBER 1: A damaged police vehicle and U.S. Postal Service van blocked off with tape nearby to where a gunman was shot and killed at Cinergy Odessa movie theater following a deadly shooting spree on September 1, 2019 in Odessa, Texas. Seven people had been killed, in addition to the gunman and at least 21 others were wounded, including three law enforcement officers after a gunman went on a rampage. The man who has not been identified fled from state troopers who had tried to pull him over. The gunman then hijacked a United States postal van and indiscriminately fired from a rifle at people before the authorities shot and killed him outside a movie theater in Odessa. (Photo by Cengiz Yar/Getty Images) ODESSA, TX - SEPTEMBER 1: A damaged police vehicle and U.S. Postal Service van blocked off with tape nearby to where a gunman was shot and killed at Cinergy Odessa movie theater following a deadly shooting spree on September 1, 2019 in Odessa, Texas. Seven people had been killed, in addition to the gunman and at least 21 others were wounded, including three law enforcement officers after a gunman went on a rampage. The man who has not been identified fled from state troopers who had tried to pull him over. The gunman then hijacked a United States postal van and indiscriminately fired from a rifle at people before the authorities shot and killed him outside a movie theater in Odessa. (Photo by Cengiz Yar/Getty Images) ODESSA, TX - SEPTEMBER 1: Texas Gov. Greg Abbott leaves a press conference with local and federal law enforcement at the University of Texas of the Permian Basin (UTPB) following a deadly shooting spree on September 1, 2019 in Odessa, Texas. Seven people had been killed, in addition to the gunman and at least 21 others were wounded, including three law enforcement officers after a gunman went on a rampage. The man who has not been identified fled from state troopers who had tried to pull him over. The gunman then hijacked a United States postal van and indiscriminately fired from a rifle at people before the authorities shot and killed him outside a movie theater in Odessa. (Photo by Cengiz Yar/Getty Images) ODESSA, TX - SEPTEMBER 1: Texas Gov. Greg Abbott holds a press conference with local and federal law enforcement at the University of Texas of the Permian Basin (UTPB) following a deadly shooting spree on September 1, 2019 in Odessa, Texas. Seven people had been killed, in addition to the gunman and at least 21 others were wounded, including three law enforcement officers after a gunman went on a rampage. The man who has not been identified fled from state troopers who had tried to pull him over. The gunman then hijacked a United States postal van and indiscriminately fired from a rifle at people before the authorities shot and killed him outside a movie theater in Odessa. (Photo by Cengiz Yar/Getty Images) ODESSA, TX - SEPTEMBER 1: Texas Gov. Greg Abbott holds a press conference with local and federal law enforcement at the University of Texas of the Permian Basin (UTPB) following a deadly shooting spree on September 1, 2019 in Odessa, Texas. Seven people had been killed, in addition to the gunman and at least 21 others were wounded, including three law enforcement officers after a gunman went on a rampage. The man who has not been identified fled from state troopers who had tried to pull him over. The gunman then hijacked a United States postal van and indiscriminately fired from a rifle at people before the authorities shot and killed him outside a movie theater in Odessa. (Photo by Cengiz Yar/Getty Images) ODESSA, TX - SEPTEMBER 1: Flowers hang on the Odessa Police Department sign following a deadly shooting spree on September 1, 2019 in Odessa, Texas. Seven people had been killed, in addition to the gunman and at least 21 others were wounded, including three law enforcement officers after a gunman went on a rampage. The man who has not been identified fled from state troopers who had tried to pull him over. The gunman then hijacked a United States postal van and indiscriminately fired from a rifle at people before the authorities shot and killed him outside a movie theater in Odessa. (Photo by Cengiz Yar/Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

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