A death row inmate who committed one of Utah's most notorious murders lost a key round of appeals Monday and inched closer to his preferred manner of execution: a firing squad.

Ronald Watson Lafferty, who claimed God told him to kill his sister-in-law and her baby daughter in 1984, could be put to death by summer 2020, according to Andrew Peterson, assistant solicitor general at Utah's Office of the Attorney General.

"If things go as I expect them to go, taking into account all the last-minute delays, I would be unsurprised if the execution were to take place next summer," Peterson told NBC News on Tuesday.

The Denver-based 10th Circuit Court of Appeals on Monday denied Lafferty's request to mount appeals on four issues — that the state of Utah had no right to retry him after his initial conviction was set aside; that his retrial violated double-jeopardy protections; that he was the victim of ineffective counsel; and that he was wrongly deemed to be competent to stand trial.

“This was his remaining last challenges to his conviction and sentencing," Peterson said. "There are no other avenues that I am aware of to challenge the judgment against him."

Lafferty, 78, could still ask the 10th Circuit to reconsider its decision or ask the full court to hear his case.

Dale Baich, Lafferty's lawyer, said in a statement that he'll use all options to challenge his client's conviction and death sentence, vowing to go all the way to the U.S. Supreme Court if necessary.

"When the most severe penalty a state can impose is at stake, we look to the courts to be the safety net to ensure that the full protections allowed by the Constitution have been met," Baich said in a statement. "Here, the court relied on procedural technicalities to deny Mr. Lafferty a complete appellate review of his case."

Lethal injection is Utah's primary method of execution, but any prisoner sentenced before May 3, 2004, could opt for the firing squad.

While Lafferty has already elected to face a firing squad, Peterson left the door open for the killer to possibly change his mind.

"Whether anyone would oppose it, whether the Department of Corrections or our office would oppose it, that would be another question," Peterson said.

There have been seven executions in Utah since capital punishment was temporarily halted and then reinstated in the 1970s. Utah was the first state to put a prisoner to death following that moratorium, when Gary Gilmore was executed by firing squad on Jan. 17, 1977.

Lafferty was convicted of killing his sister-in-law Brenda Lafferty, 24, and her 15-month-old daughter, Erica, in crimes captured in Jon Krakauer’s popular book about radical offshoots of The Church of Jesus Christ of Latter-day Saints, "Under the Banner of Heaven."

He claimed to have been acting on orders from God to kill them because of Brenda Lafferty's opposition to his fundamentalist beliefs in polygamy.