One of the suspects who shot up a Kansas City bar Sunday, killing four customers and injuring five others, flipped out after being turned away just a few hours earlier, according to staff.

The unidentified man — who had yet to be arrested by Sunday afternoon — had previously caused trouble at the Tequila KC Bar and was refused service when he walked in at 11 p.m. Saturday, barman Jose Valdez told the Kansas City Star.

The furious customer stormed out after throwing a cup at Valdez — only to come back with a handgun at 1:30 a.m. Sunday and open fire, killing four men and injuring five others, police say. He had at least one other man with him.

They were able to get in because the bar’s security guard was out sick, Valdez said.

No arrests had been made by Sunday afternoon, cops said.

“How can you go into a place full of people and just start shooting?” Valdez asked the paper, choking up as he said more than a dozen shots produced so much smoke, he thought the building was “going to cave in.”

“I don’t know what to make of it. A sad day for everybody who lost their lives and their families,” he said. “We lost some good people, young people.”

Witness Toni Maciel, 36, said she knows one of the suspects’ wives and said he had been “persistent” when he was kicked out of the bar, which she called a “home away from home” for most regulars.

“He left because they kind of kicked him out, but he wouldn’t leave,” Maciel told the Star. “It took about four people to kick him out. After that, maybe an hour or two later, he came back and this happened.

“They took a brother, a dad, a son, somebody. It’s not right.”

Customer Michael Barajas, who was hit in the shoulder, said he was too shocked to move quickly when the gunfire erupted.

“They went off so quick, I didn’t think it was gunshots,” he told the paper, saying he texted his fiancee “I’ve been shot” as he finally ran out.

Juan Ramirez said his nephew, 29-year-old Alfredo Calderon, was among the dead.

The nephew, a father of two young children, had “nothing to do with it,” he said, a tear running down his face.

“We’re just in shock and disbelief. I don’t wish this upon anybody.”