MISSOULA, Montana (REUTERS) - A Montana homeowner who was found guilty of deliberate homicide last year for fatally shooting an unarmed 17-year-old German exchange student who entered his garage was sentenced to 70 years in prison on Thursday.

Markus Kaarma will not be eligible for parole for the first 20 years of his sentence.

He was convicted in December of killing Diren Dede of Hamburg in a trial that tested Montana's so-called "castle doctrine" self-defence law, which allows deadly force against a home invasion if a person reasonably believes it is necessary to prevent an assault.

At trial, prosecutors painted Kaarma, 30, as an armed aggressor who lured Dede to his death while the student was"garage hopping" at night in Missoula, perhaps looking for alcohol.

The Missoula County District Court heard how Kaarma had installed motion detectors and a baby monitor days before the shooting, and deliberately left a purse filled with cash and other items in the garage on the day he killed Dede.

Prosecutors said Kaarma lost legal protection for his actions under the state's law when he left his house to corner the student in the garage after being alerted to his presence by the monitoring devices.

They also cast doubt throughout the trial on whether Kaarma believed any danger existed, and said ballistic evidence showed that, after wounding Dede, he had repositioned himself for a final shot to kill the unarmed teen.

Defence attorneys countered unsuccessfully that Kaarma was under no obligation to retreat from an intruder, and that his actions were in line with Montana law.

The defence lawyers had also called for a new trial, making the argument that "prejudicial" media coverage of the case had made it impossible to seat an impartial jury.

Judge Ed McLean rejected that request, and another that Kaarma's conviction be downgraded from deliberate homicide, during a hearing earlier on Thursday before the sentence was delivered.