MOORHEAD, Minn. (AP) — A furnace was the source of carbon monoxide that last month killed seven members of an immigrant family from Honduras living in Minnesota, authorities said Monday.

Police in Moorhead said earlier that blood samples from the victims showed a lethal level of carbon monoxide, but further testing was needed to determine whether the source was the furnace or a van parked in an attached garage. New findings from the Ramsey County Medical Examiner’s Office showed that the poison gas could not have come from a combustion engine.

The victims were found the night of Dec. 18 by family members conducting a welfare check at the home in south Moorhead.

Investigators found that a carbon monoxide detector in the garage had been removed and replaced with a smoke-only detector. An initial investigation by technicians did not reveal a defect in the furnace that would have sent carbon monoxide into the home.

The family members who died were identified as Belin Hernandez, 37; Marleny Pinto, 34; Eldor Hernandez Castillo, 32; Mariela Guzman Pinto, 19; Breylin Hernandez, 16; Mike Hernandez, 7, and Marbely Hernandez, 5. They all lived together, police said.

Moorhead police said they have concluded the investigation.

Moorhead is on the Minnesota border next to Fargo, North Dakota, in a metropolitan area of about 230,000 people.