Pilot error and a lack of proper warning equipment is being blamed for the 2008 crash of a University of Wisconsin Med Flight helicopter that killed three people.

The National Transportation Safety Board announced its findings in the final report on the crash released Friday.

The UW Med Flight helicopter crashed into a hilly, wooded area about 4 miles from the La Crosse airport. The crash killed pilot Steve Lipperer, nurse Mark Coyne and doctor Darren Bean. The 86-year-old patient who was transported died hours later.

The NTSB report blames the crash on inadequate preflight planning that resulted in Lipperer's failure to clear trees on top of the ridge. It also says there wasn't an onboard warning system that would alert him to the approaching trees.