KANSAS CITY, Kan. (AP) — Authorities say carbon monoxide poisoning killed two people who were found dead in a sport utility vehicle in a Wal-Mart parking lot in Kansas City, Kansas.

Police Chief Terry Zeigler announced the cause of death Tuesday on Twitter. The names of the two people who died weren’t immediately released. They were found Sunday morning in the parking lot across the streets from the Legends Outlets shopping center near the Kansas Speedway.

Zeigler said in a later tweet that he wasn’t sure what caused the carbon monoxide levels to reach lethal levels. He said “an exhaust issue” was a possibility.