Updated

A woman and her two young children were among seven people found dead after a "strange odor" was reported coming from a duplex apartment, and a third child was unaccounted for, police said Monday.

The cause of death had not been established, and police Chief Jim Hughes would not speculate on the possibility of foul play.

The chief said a fire department sensor indicated a high level of carbon monoxide, but the home heating system was functioning properly and was not the source of any problem.

"It's a very critical element of our investigation," Hughes said.

He would not say whether a van parked in the garage was running when police arrived, but said it was taken to a crime lab. He said he expected autopsy results no later than Tuesday.

Police did not immediately identify the victims, but said they were a 23-year-old resident of the apartment, her 23-month-old daughter and her 1-year-old son, and a woman and three men ages 19 to 21, officials said.

The woman also had a 4-year-old child who lived there, but that child was not at home and has been accounted for, Hughes said. Police weren't sure how many people lived in the apartment and were trying to determine the victims' relationships.

Paramedics found the bodies Sunday afternoon after 911 callers reported "a number of unresponsive individuals ... and a strange odor" inside the home, Hughes said.

Authorities weren't sure exactly how long those inside had been dead, but they believe the victims were found within 24 hours. The victims were found in three different rooms.

The yellow duplex sits on the corner of a quiet cul-de-sac a few blocks from the Truman State University campus in Kirksville, about 165 miles northwest of St. Louis.