A 48-year-old man was sentenced Wednesday to life in prison for killing his daughter-in-law and her 1-year-old son and setting their apartment on fire.

Takemitsu Mori, an Osaka Prison guard, denied committing murder and arson throughout the investigation and in his Osaka District Court trial. Prosecutors built their case solely on circumstantial evidence and demanded the death sentence.

According to the court, Mori killed Mayumi Mori, 28, and her son, Toma, on April 14, 2002, and set fire to their apartment. At the time, Mori was married to the mother of Mayumi's husband but later got divorced.

Presiding Judge Masanori Tsunoda said evidence presented by prosecutors -- test results that showed Mori's DNA matched that of saliva taken from a cigarette butt found on a stairwell at the apartment complex and witness accounts of a man resembling him being near the scene -- was credible.

"The court recognizes that the defendant was at the apartment that day," he said.

At the same time, Tsunoda said Mori's account of what he did that day could not be trusted, as his explanations were vague, and at times he changed them.

Tsunoda also said he was hesitant about sentencing Mori to death, noting that the court saw no premeditation in the murder.