On Thursday, the State Police said that their investigation of the accident was continuing and forwarded questions about the N.T.S.B.’s letter to Schoharie County officials. Ms. Mallery did not respond to requests for comment; her office said she was in meetings.

In their more recent letter, federal officials seem exasperated by local intransigence, adding that their pleas repeatedly went unacknowledged. “What we have been told is that your schedule is full,” Ms. Silbaugh wrote, “and you are too busy to respond.”

The seeming impasse in the investigation is in stark contrast to the violence of the accident and the flurry of developments in its immediate aftermath, including the revelation that the owner of the limousine company that rented the vehicle, Prestige Limousine, was Shahed Hussain, a former F.B.I. informant with a criminal record who has been living in Pakistan.

Shortly after the accident, Mr. Hussain’s son Nauman Hussain, the operator of the company, was charged with criminally negligent homicide. He pleaded not guilty.

Ms. Mallery’s office is in charge of prosecuting Nauman Hussain. Because of that, Ms. Silbaugh said, federal authorities have allowed Schoharie County first access to the evidence.

The N.T.S.B. is an independent federal agency that investigates major transportation accidents. Its mandate is to recommend improvements; it does not have the authority to prosecute.

But while some information has been shared with the safety board, including state transportation reports and information on the victims’ families, Ms. Silbaugh wrote that access to “primary, essential evidence” had been delayed, resulting in “safety-critical evidence being lost.”