SUPERIOR, Wis. (AP) -- A cause of a July explosion in northwestern Wisconsin that killed a woman and severely injured her two sons could still take months.

Authorities have said the incident appeared to have been the result of a propane explosion from one of the appliances.

Greg St. Onge is a fire investigator with Ember Investigations of Brule who was retained by the family's insurance company to investigate.

He says it's hard to say when a definitive cause will be found.

Among those investigating are various experts, investigators and insurance company representatives.

Fifty-four-year-old Bridgette Arneson of Chaska, Minnesota died and her sons 16-year-old Ethan Cline and 15-year-old Colin Cline were injured.

(Copyright 2007 by The Associated Press. All Rights Reserved.)