(CBS) – Two 18-year-olds found in a car last month in west suburban St. Charles died from carbon monoxide poisoning, possibly after a “do-it-yourself” repair on the vehicle’s exhaust system, police said.

Authorities found the bodies of Kody Deitrich of St. Charles and Adalina Donoso of Racine, Wisconsin, in the back seat of a 2001 Toyota Camry on March 16 off 17^th Street.

The Kane County Coroner’s Office determined both died from carbon monoxide intoxication. St. Charles police say that is consistent with their own investigation showing the car generated dangerously high levels of CO.

Police say the Camry’s manifold was not connected to the exhaust tailpipe, allowing potentially deadly gas to empty into the engine compartment.

“This was the result of recent do-it-yourself work performed on the car,” police said in a news release issued Tuesday.

Police added: “There is no indication this is anything other than a very tragic incident.”