A Montana homeowner is facing a murder charge after he fatally shot a teenager who had entered his garage.

The 30-year-old man, Markus Kaarma, pleaded not guilty and is expected to claim the "castle doctrine" as his defense. 

The law allows a homeowner to use deadly force against an intruder when they have reason to believe they or others are at risk for bodily harm.

In a twist to the case that may test the limits of that law, Missoula County prosecutor Jennifer Clark told the jury that Kaarma left a purse and other items in his open garage to lure burglars after previous break-ins.

Authorities say 17-year-old Diren Dede, an exchange student from Germany, was killed when he entered the garage at night in a possible pursuit of alcohol. 

Prosecutors say Kaarma also installed motion detectors and a baby monitor in the days before the fatal encounter in April. 

More from Reuters:

On the night of the shooting, Dede and a fellow exchange student approached Kaarma's open garage, and Dede entered, while his friend remained on the street, the friend told police in an affidavit.

Alerted to Dede's presence by motion sensors and a video monitor, Kaarma waited 23 seconds after seeing an intruder in his darkened garage before grabbing his shotgun, followed by his common law wife carrying a baseball bat, Clark told the jury.

"He trapped him in the garage. At that point he became the aggressor," she said. “You can use deadly force in an occupied structure if there is a threat of violence, but you cannot be the aggressor.”

Kaarma fired four shotgun blasts, killing Dede, according to prosecutors. Clark said that in the moments before the shooting, the wife heard Dede say “No! No! No, please!”

Defense attorney Paul Ryan countered that Kaarma, a former U.S. Forest Service firefighter, acted in self-defense, fearing for the safety of his family as Dede entered a garage attached to his home.

Ryan blamed lack of police response and growing anxiety from two previous burglaries for his client's volatile behavior, and said Dede was part of a larger high school burglary ring.

Kaarma, who Ryan said suffered from a social anxiety disorder, could face 10 to 100 years in prison if convicted.

Former prosecutor Fred Tecce and defense attorney Ashleigh Merchant weighed in on the case on "Happening Now."

They reacted to testimony from a hairdresser who told the court that Kaarma said he was "really tired and that he'd been sitting up for three days waiting to shoot some effin' kids."

Tecce said he doesn't even see this as a tough case for the state, believing Kaarma will be convicted because there appears to have been premeditation. 

"There's a line and he stepped over it," said Tecce.

Merchant said the hairdressers' testimony really calls into question the intent of Kaarma on that night. 

She also noted that typically in Montana this would be a very tough case for a prosecutor to prove.

Watch the discussion above.