Officials say both fires that burned 6,500 acres and forced thousands out of their homes this week were caused by downed power lines.
Xcel Energy says falling trees downed the lines south of Denver and northwest of Boulder on Wednesday.
Spokesman Mark Stutz says 80 mile per hour winds were recorded in the area. He says both power lines were properly maintained.
But the state wants to investigate whether lax maintenance was a factor.
Gov. Bill Owens says the company could be fined, depending on the results of the investigation.
Freezing rain and cooler temperatures allowed firefighters to get a handle on the two fires Thursday.
Evacuated residents in the Castle Pines area of Douglas County were allowed to return to their homes on Thursday morning.