ATLANTA — The Georgia Supreme Court on Monday upheld the 2010 murder convictions of an Effingham County man sentenced to life in prison for the shotgun slayings of his father and brother after they discovered the inmate was having an affair with his brother’s wife.

Attorneys for Craig Heidt, 44, had urged the state’s highest court to order him a new trial. They said prosecutors ignored evidence of a 12-guage shotgun that may have pointed to someone else as the killer of real estate developer Philip Heidt and his son, Carey Heidt.

The court unanimously ruled against Heidt, who had also claimed the evidence against him was insufficient to sustain a conviction. Prosecutors argued the shotgun being cited by Heidt’s attorneys belonged to his slain brother and had been ruled out as a possible murder weapon before the trial.

“Heidt failed to come forward with any evidence that the state failed to disclose the existence of the shotgun,” Justice Keith R. Blackwell wrote in the state Supreme Court’s opinion.

The victims were hit with 12-guage buckshot, but no gun was ever presented to the jury as evidence. The killer also left no shell casings.

Heidt’s attorneys said a shotgun that a family friend turned over to the Effingham County sheriff during Heidt’s trial may have yielded evidence to clear him.

Sheriff Jimmy McDuffie said the gun belonged to Carey Heidt and insisted investigators had seen it before. McDuffie told a judge that both prosecutors and Heidt’s attorney were told during a trial break about the shotgun and both sides agreed it didn’t warrant delaying the case. Heidt’s attorney, W. Dow Bonds, said he could not recall such a meeting.