Stewart has told The Salt Lake Tribune he thought people were breaking into his home to rob and kill him.
"When you're convinced that you are getting robbed and most likely killed by a group of armed men, your instincts kick in," Stewart told a reporter in February.
Stewart's attorney, Randy Richards, is remaining tight-lipped about specific strategies under consideration by Stewart's five-lawyer team but said they are "looking at every avenue that would have merit."
Utah law allows a homeowner to use deadly force in defense of habitation if he "reasonably believes" the intruder intends to commit a felony in the home or there is a threat of personal violence. However, the law also states that force can be used only to prevent "unlawful" entry or attack of a person's home.
While Richards called defense of habitation an "obvious issue," prosecutors say it's not one he can bring to trial.
"I do not believe that defense is available to them legally," Deputy Weber County Attorney Gary Heward said. "Police entry was not unlawful. It was exactly the opposite."
Paul Cassell, a former federal judge and current University of Utah law professor, said his interpretation of the law also leads him to the conclusion that Stewart cannot argue he was protecting his home that night.
"It has to not only be his belief, it has to also be true," Cassell said. "It would not be a situation of defense of habitation. That particular statute does not work."
Although Stewart may not use the defense of habitation statute, two other potential defenses may be available to him, according to experts.
One strategy would be general self-defense, Cassell said.
"Ninety-nine times out of 100, the facts are quite clear that the law enforcement [officers] are there on police business," he said, referring to other cases in which a citizen shot a police officer. "What is maybe different here is the defendant is arguing that there was legitimate confusion on who were the people entering his home."
Strike force agents testified that they went to Stewart's home at around 8:30 p.m. on Jan. 4 and announced themselves before breaking through the door and entering. The agents testified they were all wearing some sort of police identifier, whether it was a jacket with the word "police" written in bold, a bulletproof vest with "police" printed on it, or a fleece vest with police insignia embroidered on the chest.
However, Stewart's attorneys have questioned whether a person would be able to understand the hollering of "Police! Search warrant!" if the officers were not yelling in unison. They have also raised questions about whether agents in the strike force who often grow beards or wear a longer hairstyle than a traditional officer because of undercover assignments would be easily identifiable as law enforcement in the dark.
Utah's self-defense statute says that a person is justified in using force against another if the person "reasonably believes" the force is necessary to defend themselves from "another person's imminent use of unlawful force."
However, the law specifically says a person is not justified in using force if he provokes the violence. And the question of who fired the first shot at Stewart's home is in dispute.
Agent Shawn Grogan testified during Stewart's recent preliminary hearing that when he stood in the hallway of Stewart's home, he saw an arm and a gun coming around the bedroom door. He said the shooter said nothing, but fired first, one of the first shots striking Grogan in the left cheek.
But Stewart allegedly told an investigator with the Weber County attorney's office that he had armed himself when he heard someone enter his home. He allegedly said he pointed his gun around the corner of his bedroom hallway, and was met with gunfire. Stewart told the investigator he didn't pull his trigger until he was shot at.
Another defense that could play out in the courtroom is "mistake of fact," when a defendant claims he mistakenly believed his conduct did not break the law.
Susanne Gustin, a Salt Lake City-based defense attorney who does not represent Stewart, said she thought self-defense might be a "viable defense" for Stewart but "mistake of fact" also could be a successful defense.
"[Stewart's attorneys] are going to have to toy with that … [that] he mistakenly believed that these people were not police officers, when in fact, they were police officers," Gustin said.
Cassell said that under the mistake of fact statute, Stewart's attorneys could either argue that he broke no law, since it is not illegal to shoot a burglar in one's own home; or he could use mistake of fact as a partial defense, asking for reduced charges because he was unsure of who was in the home but thought they might be burglars.
"[Jurors] might convict him of [the lesser crime of] manslaughter, saying you weren't sure if they were burglars, and you were reckless in that," Cassell said.
Stewart, 38, has pleaded not guilty to aggravated murder in Francom's death, seven first-degree felony counts of attempted aggravated murder for allegedly trying to kill other officers, and one second-degree felony count related to alleged marijuana cultivation.
Prosecutors have said they plan to seek the death penalty for Stewart, who is not expected back in court until a Feb. 14 scheduling hearing.
A trial date has not been set.