Kevin Dahlgren, 24, fatally slashed a man 29 times in his study before also stabbing the man’s wife and their 23-year-old son to death at their home in Brno, eastern Czech Republic.
Brno regional court judge Michal Zamecnik ruled that Dahlgren suffered from emotional instability, narcissism, paranoia and increased aggression.
Following the frenzied assault, he attacked the dead couple’s 17-year-old son, striking him on the head with a stone as he returned home from school before stabbing him six times, said the judge.
He then tried to burn the bodies of his victims inside the house. Czech media said the dead woman was Dahlgren’s cousin.
Dahlgren claimed he had heard “a voice” telling him to attack his family. His lawyer has lodged an appeal against the sentence.
Following the incident Dahlgren flew to Vienna and on to the United States where he was detained at a Washington airport the day after the killings.
Last year he became the first American to be extradited to the Czech Republic, an EU member of 10.5 million people.
Dahlgren’s Facebook page said he was from Palo Alto, California, and that he had offered English lessons in Brno, the Czech Republic’s second-largest city.