MADISON, Wis. -- Federal investigators said fog might have contributed to the fatal crash of a University of Wisconsin Med Flight helicopter last year.

Documents released Thursday said that two medical helicopter crews in the area told federal safety investigators they declined to fly on the evening of May 10 because of poor weather.

One pilot said he noticed fog forming on bluffs near the Mississippi River during an earlier flight.

The UW Med Flight helicopter crashed into hilly terrain after leaving the La Crosse airport. The crew was returning to Madison after dropping an 86-year-old patient off at a hospital in La Crosse.

The crash killed pilot Steve Lipperer, nurse Mark Coyne and doctor Darren Bean. The patient died hours later.

Air Methods operated the UW Med Flight craft. Its spokesman said the weather was "legal and safe" at the time of the flight.

The National Transportation Safety Board says it has not found a probable cause of the crash. The board released hundreds of documents related to its investigation on Thursday.

In a statement Thursday, UW Health said it hopes the final report to be released in the coming months will shed more light on the cause of the crash.

UW Health spokeswoman Toni Morrissey said that despite Med Flight not operating for approximately two weeks after the crash, the program still reported 1078 flights for 2008.

Morrissey said Med Flight has flown a total of 745 flights from the time of service resumption to Thursday. If present trends continue, UW Health said the Med Flight air service will be very close to the number of flights logged in the year immediately before the May crash.

Morrissey also said that the stress of the tragic crash has led some personnel to leave the program and move into different roles within UW Health. She said all pilots, dispatchers, most nurses and flight physicians have stayed with the program, but six physicians have opted not to fly, some citing family considerations.