The West Texas man who killed seven people and wounded 25 in a shooting rampage bought his gun in a private sale, which doesn't require a background check, a law enforcement official told CNN.
Police have described the firearm Seth Ator, 36, used in Saturday's killing spree in and around Odessa, Texas, as an AR-15-style rifle.
Ator tried to buy a firearm on January 14, 2014, but wasn't allowed to, the Texas Department of Public Safety told CNN in an email.
The department is prohibited by law from disclosing the reason he was denied the purchase, the DPS said.
Ator failed a background check when he applied to get a gun, a representative for the Bureau of Alcohol, Tobacco, Firearms and Explosives said Monday, without giving details.
The ATF, the FBI and the DPS are "aggressively following up on" the source that supplied the firearm to Ator, the ATF representative said.
Investigators are still searching for a motive in the mass shooting.
The man showed up to work Saturday "in a distressed mental state," and was fired, said FBI Special Agent in Charge Christopher Combs.
Speaking at a press conference Monday, Combs said the firing wasn't the reason for the shooting spree and that he was "already enraged" when he showed up for work.
Ator had been "on a long spiral going down," and he had "probably been in trouble for a while," Combs said.
The shooting began Saturday when Texas Department of Public Safety troopers pulled Ator over for failing to use his turn signal, and Ator began firing at them. Speeding away, he went on to spray bullets as he drove west toward Odessa and into the city. At one point, he hijacked a US Postal Service truck, killing the mail carrier.
Ator was finally stopped in a parking lot after officers rammed the hijacked truck and gunned him down.
"The reason that person was stopped was because of a Midland police officer and an Odessa police officer. They rammed his car, stopped him and when he got out, they shot him," Odessa Mayor David Turner said.