DENVER (AP) -- Investigators are studying weather conditions at the time a single-engine airplane crashed in the southern Colorado mountains and killed a doctor and a retired businesswoman from Iowa.

The National Transportation Safety Board disclosed the study in a preliminary report late Wednesday.

The report doesn't suggest any possible causes for the crash, including weather.

The pilot, 66-year-old Michael O. Welton, and 70-year-old Roswitha Marold of Waterloo, Iowa, were killed when the Piper Malibu crashed Jan. 9.

The wreckage was found the next day about 110 miles southwest of Denver.

Search-and-rescue officials say the wreckage was difficult to find in part because the plane appeared to fall nearly vertically, leaving few damaged trees.

(Copyright ©2011 by The Associated Press. All Rights Reserved.)