(CNN) The 36-year-old gunman who killed seven people in a West Texas shooting rampage Saturday was already in a "distressed mental state" when he showed up to work, and was fired just hours before the massacre began, the FBI said.

The shootings "did not happen because he was fired," said Christopher Combs, FBI special agent in charge, at a press conference Monday. "When he showed up to work, he was already enraged."

Seth Ator, the shooter, had applied to get a gun but failed a background check, a representative for the Bureau of Alcohol, Tobacco, Firearms and Explosives said at the press conference.

Ator bought the weapon used in the shootings in a private sale, which doesn't require a background check, a law enforcement official told CNN Tuesday.

The ATF, the FBI and the DPS are "aggressively following up on" the source that supplied the firearm to Ator.

A photo of Seth Ator released by the city of Odessa Monday.

The FBI has been searching a property in West Odessa linked to Ator, and Combs called it a "very strange residence" in conditions that "reflect what we believe his mental state was going into this."

Ator, Combs said, was "on a long spiral of going down."

"He's probably been in trouble for a while," Combs said.

Both Ator and his employer, Journey Oilfield Services, called 911 right after the firing, Odessa Police Chief Michael Gerke said.

"Basically, they were complaining on each other because they had a disagreement over the firing," Gerke said at a press conference Monday.

FBI agents search a home believed to be linked to Ator in West Odessa, Texas.

Shortly before being stopped by a Department of Public Safety trooper, Ator also called the FBI national tip line, but he made "no threats of any type of violence," Gerke said.

That changed during the shooting spree, when Ator called 911 twice. In those calls, he was telling the dispatcher that "he's the guy doing it," Combs said.

Authorities have repeatedly cautioned that they have yet to establish a motive for the shooting. They say they believe Ator acted alone.

Ator began spraying the roads with bullets after police pulled him over for failing to use his signal. He killed seven people and wounded 25 others before authorities rammed the hijacked mail truck he was driving 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.

The number of injured rose by three to 25 Monday. One of those was a 70-year-old woman was struck by shrapnel while in her car and went to a local hospital, where she was treated and released, Gerke said.

'Rambling statements'

Ator's call to the FBI before his encounter with the troopers included "rambling statements about some of the atrocities that he felt he had gone through," said Christopher Combs, FBI special agent in charge.

The troopers who stopped Ator for a traffic violation Saturday did not know about the calls.

Those who were killed ranged from 15 to 57 years old, Gerke said, and included Mary Granados , a 29-year-old mail carrier who was on the phone with her twin sister as she neared the end of her shift.

Also killed was Kameron Brown, 30, who worked for Standard Safety & Supply, the company said. The company linked to a GoFundMe page for Brown, which said he was a resident of Brownwood, Texas, and had served in the Army in Afghanistan.

The five others killed were a 15-year-old girl; Joe Griffith, 40, Edwin Peregrino, 25, and Rodolfo Julio Arco, 57, all of Odessa; and Raul Garcia, 35, of El Paso, the City of Odessa said Monday. The city did not release the name of the slain teenager.

A 17-month-old girl and three law enforcement officers were among the injured.

He was pulled over for not using his signal

The shooter was pulled over by Texas troopers in Midland on Saturday afternoon, police said.

He began shooting at officers with what police described as an assault-type weapon and sped away. He continued firing randomly at residents and motorists, police said.

He then hijacked a postal truck and made his way to Odessa, about 20 miles away. That's where police confronted him in a parking lot and killed him.

FBI agents search a home believed to be linked to a suspect following a deadly shooting spree on September 1, 2019 in West Odessa, Texas.

A police vehicle rammed the truck carrying the gunman, sending it spinning into a group of cars. Authorities surrounded him, according to bystander video and Midland County District Attorney Laura Nodolf.

"Law enforcement at that point had collectively taken efforts to surround him, and he was not going to go anywhere, and he was not going to hurt anybody else," Nodolf said. "That is the definition of heroism, when you have people you know are walking into fire and could be hit."

Investigators are combing through 15 crime scenes and multiple cars, the FBI's Combs said.

At this point federal investigators do not believe there's "any connection to any domestic or international terrorism," Combs said.

The victims

Granados' sister, Rosie, said she heard her sister scream while they were on the phone.

"It was very painful," she said. "I just wanted to help her and I couldn't. I thought she had gotten bit by a dog or something. I tried calling her name and she wouldn't answer."

The Ector County Independent School District in Odessa said one of its students was among those killed.

The 17-month-old who was injured was airlifted to University medical Center in Lubbock, where she underwent surgery, and she has since been released and is resting at home, Ector County Sheriff Mike Griffis said, citing a message he received from the toddler's grandfather.

A 17-month-old girl was injured by gunfire on Saturday in Odessa, Texas.

The girl is expected to make a full recovery, her family said.

"We hurt so badly for the families whose loved ones didn't survive this tragedy," her parents, Kelby and Garret Davis, said in a statement.

The law enforcement officers wounded were a state trooper, a Midland officer and an Odessa officer, officials said. The trooper was in serious condition Sunday, and the other officers were in stable condition.

The Odessa officer, James Santana, is expected to be released from the hospital Tuesday, Odessa police Cpl. Steve LeSuer said.

Number of active shooting incidents increasing

More than 40 people have died in the four shootings in Texas Ohio and California since July. It's part of a trend the FBI says is getting worse each year.

"If you look at the numbers, we're looking at an active shooter every other week in this country," Combs said.

The agency has conducted a study on all active shooters -- who they define as "an individual actively engaged in killing or attempting to kill people in a populated area" -- between 2000 and 2018 and found the numbers keep increasing.

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"I think it is frustrating for all of us in law enforcement that we keep having to do this," he said, referring to ongoing investigations. "For the FBI in particular, we do them across the country as a service to our state and local partners. It's just getting worse."

"I have been to too many of these events," Gov. Greg Abbott said. "I am heartbroken by the crying of the people in the state of Texas. I am tired of the dying of the people of Texas. Too many Texans are in mourning. Too many Texans have lost their lives. The status quo in Texas is unacceptable, and action is needed."