Investigators discovered “overwhelming evidence" that a Massachusetts mother and her three young children were murdered by their father last week before he set fire to their home and committed suicide, officials said Tuesday.

Luke Karpinski and his wife, Justine Wilbur, both 41, and their children, a set of 7-year-old twins and a 3-year-old, were found dead inside their Sheffield home March 13 when firefighters arrived to put out the blaze, which appeared to have been intentionally set.

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Investigators located evidence of an accelerant in different parts of the home, including two 20-pound propane tanks on the upper floor, where Karpinksi and the children were discovered, Mass Live reported.

“This investigation is complex and ongoing, but we have uncovered overwhelming evidence suggesting that Luke Karpinski killed his wife and children prior to committing suicide,” District Attorney Andrea Harrington said.

Wilbur, whose body was found separated from her children and Karpinski, showed signs of a “traumatic injury” that occurred before the fire was set, Harrington said. Police did not find any evidence of guns in the home, she said.

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The Berkshire Eagle, citing an unnamed source, reported Saturday that Karpinksi may have stabbed his family to death before starting the fire.

Authorities have not confirmed the family’s cause of deaths. Autopsy results for the couple and their children were pending as the investigation continued.

Investigators have not disclosed a possible motive for the killings.

The Associated Press contributed to this report.