Johnnie Langendorff stumbled into the crossfire — a total accident.

Sunday morning was all routine until then. Langendorff — a lanky Texan with a fuzzy chin beard and the long horns of a bull’s skull tattooed across his neck — had breakfast. Then he was driving his truck on the dusty back streets to his girlfriend’s house nearby. When he approached the First Baptist Church of Sutherland Springs, he noticed something odd.

As he passed the churchgoers’ cars parked around the white wood front of the building, he saw that one vehicle’s engine was running. It was a pearl-colored SUV, a Ford Explorer, he said. The driver’s door was open. A man clad all in black was walking toward the vehicle with a pistol. He was trading shots with another man holding a rifle.

“I never got a look at him,” Langendorff explained to reporters later, his mouth nervously working a toothpick, when asked about the black-clad shooter. “I never really saw him. I saw the gunfire.”

The man in black hauled off in his SUV. The second man with the rifle — a neighbor unidentified as of Sunday night — came to Langendorff. The two men were strangers. “He briefed me quickly on what had just happened and said he had to get him,” Langendorff said later. “So that’s what I did.”

Johnnie Langendorff speaks to reporters about the mass shooting at the First Baptist Church in Sutherland Springs, Texas, on Monday. Langendorff said he and another man chased down the gunman after he fled the church where he killed at least 26 people.

As the two men shot off in pursuit in Langerdorff’s truck, 26 First Baptist worshipers were dead or bleeding out on the sanctuary’s maroon carpet. Dozens were wounded. Sunday’s burst of violence would later be recognized as the fifth worst shooting in modern U.S. history, arriving just a month after 58 concertgoers were murdered in Las Vegas.

The Texas’ victims’ ages ran from 5 to 72, according to authorities, though one family said a 1-year-old died. The shooter, identified now as Devin Patrick Kelley, first opened fire on the outside of the church just after 11 a.m. with a Ruger assault rifle. Kelley, clad in black tactical gear, then sprayed the building’s inside with bullets. He hastily fled the scene after engaging in a gun battle with a neighbor, the same man subsequently riding shotgun in Langendorff’s truck.

“It was more see and do,” Langendorff later told reporters. “Act now, ask questions later.”

Blasted with adrenaline, Langerdorff wove his truck at high-speed through traffic while trying to catch the fleeing car. The speedometer crossed 95 mph while the driver narrated everything to law enforcement. “I was on the phone with dispatch the entire time,” he said. “I gave them the direction we were going, on what road and everything, and that the vehicle was in sight and that I was getting closer and closer to him.”

Kelley’s vehicle, however, broke from the roadway and crashed into a ditch about 11 miles north of the church. Langerdorff pulled his own truck within 25 yards.

“The gentleman that was with me got out, rested his rifle on my hood and kept it aimed at him, telling him to get out get out. There was no movement, there was none of that. I just know his brake lights were going on and off, so he might have been unconscious from the crash or something like that, I’m not sure,” he said.

Police were on the scene within 5 to 7 minutes, Langerdorff said Sunday night. Freeman Martin, a regional director for the Texas Department of Public Service, told The Post authorities had yet to determine whether Kelley was killed by a self-inflicted gunshot wound or hit in the gunfire at the church. Multiple weapons were found in Kelley’s vehicle.

On Sunday night, Langendorff explained his reaction to the shooting — jumping into a car chase — was a simple calculation. “He just hurt so many people, he affected so many people’s lives, why wouldn’t you want to take him down.”