Federal investigators say they've found no evidence that mechanical failure was the cause of a medical helicopter crash earlier this month that killed a 1-year-old patient and three crew members.

The helicopter's flight controls, engines and drive system showed no sign of malfunction just before the Oct. 15 crash in the Chicago suburb of Aurora, the National Transportation Safety Board said in a preliminary report released Thursday.

The report does not address what might have caused the late-night accident, but at least one helicopter safety expert said the NTSB narrative leaves little doubt pilot error is to blame.

The helicopter clipped a radio tower guy wire, crashed into a nearby field and burst into flames. The NTSB has said that still images from a security camera a few miles away from the crash site show the tower's upper and lower lights were flashing at the time.

"This is a clear case of a pilot losing situational awareness, not realizing where he was," Gary C. Robb, an aviation lawyer in Kansas City, Mo., said Thursday after reading the NTSB's report. "This is a clear pilot mistake."