A key witness in the prosecution of a demolition contractor accused of shoddy practices that led to the death of six people in a 2013 building collapse said he gave repeated warnings that the job was unsafe. 

Plato A. Marinakos, who is testifying under a grant of immunity, said he urged Griffin Campbell to dismantle a freestanding, four-story brick wall that loomed over a one-story, occupied thrift store on 21st and Market as quickly as possible.

Marinakos' testimony came in the third day of Campbell's murder trial.

RELATED:   Crane operator in '13 building collapse pleads guilty

Prosecutors say they will present evidence that Campbell was warned several times about the dangers of the wall, and that he ignored those warnings because he put profit over people. 

Marinakos said he called to make sure the wall had come down, and that Campbell had lied. 

But hours later, Campbell called with news that changed both of their lives.

"I'll never forget that call," Marinakos said. "He said, 'Plato, the building collapsed.' I though maybe a beam fell. I didn't know what to believe."

The collapse sent bricks tumbling onto the roof of the Salvation Army store, burying shoppers and employees in rubble. In addition to the six killed, 13 were injured. 

Marinakos rushed to the scene, and tried to make his way to the site amid street closures. 

"It will haunt me for the rest of my life," he said. "It was the most horrible sight." 

Marinakos said that Campbell later asked to borrow money because his insurance payment was past due. 

RELATED: Defense: contractor is a small fry

That request — Marinakos' attorney William Hobson argued — underscored the defense's contention that Campbell was not in charge of demolition. Hobson has said in opening arguments that Campbell was beholden to Marinakos. 

Marinakos' testimony ended for the day when a jury member became sick, but not before an almost macabre scene unfolded in the courtroom. 

In front of the jury on a card table stood a scale model of the Hoagie City store that was demolished, dwarfing the Salvation Army thrift store next to it. While fumbling with exhibits, Hobson nearly knocked the model over, sending gasps through the courtroom.