ALBANY, N.Y. – More than two months after a limousine crash in upstate New York killed 20 people, federal investigators still can't get access to the vehicle to complete their probe.

The National Transportation Safety Board released a letter Thursday to the Schoharie County district attorney expressing frustration with stonewalling from that office and New York State Police, saying the delay jeopardizes the investigation and leaves evidence in in peril.

"While we understand the important duties you are fulfilling, we are gravely concerned about your lack of responsiveness to our requests that have seriously impeded our abilities carry out our congressionally mandated duties to properly complete this safety investigation and potentially prevent similar accidents in the future," the the board's general counsel, Kathleen Silbaugh, wrote Dec. 14.

Seventeen passengers, the limo driver and two pedestrians were killed Oct. 6 on a rural road in Schoharie, about 30 miles west of Albany, when the vehicle crashed as occupants were on their way to celebrate the 30th birthday of one of the passengers.

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It was the deadliest transportation accident in the United States since 2009.

The NTSB immediately began investigating the cause and circumstances around the crash, but soon after it complained that New York investigators were limiting federal agents' access.

The state is conducting its own criminal investigation and already has charged Nauman Hussain, who ran Prestige Limo in the Albany area, with criminally negligent homicide. The criminal case, plus lawsuits from family members of the victims, has muddied the investigation, the federal transportation board suggested in its three-page letter.

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Thefirst lawsuit was filed last month. State Police deferred comment to Schoharie County District Attorney Susan Mallery, who did not immediately provide one.

By not having the ability to inspect the limo, the federal agency said its investigators could lose the opportunity to accurately determine what happened. Any corrosion on the vehicle or its parts and the status of the limo's electrical system at the time of the crash would help ascertain that.

The limo had a series of failed inspections before the crash.

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"During this two-month period, key perishable safety investigative information may have been lost because you denied the NTSB the necessary access," the agency wrote.

The NTSB said it first wrote Silbaugh on Oct. 17 requesting access and information about the crash but contended she has been "unresponsive."

"Instead, what we have been told is that your schedule is full and you are too busy to respond," the letter said.

Follow Joseph Spector on Twitter: @GannettAlbany

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