A federal probe into the upstate limo crash that killed 20 people has stalled because local investigators won’t yield the vehicle to inspectors, according to reports.

Schoharie County District Attorney Susan Mallery is refusing to hand over the smashed-up stretch Ford Excursion to the National Transportation Safety Board.

“…[W]e are gravely concerned that your lack of responsiveness to our requests has seriously impeded our abilities to carry out our congressionally-mandated duties to properly complete this safety investigation and potentially prevent similar accidents in the future,” NTSB attorney Kathleen Silbaugh wrote to Mallery in a scathing Dec. 14 letter obtained by WGRB-TV.

Malley said she won’t let the feds into the driver’s seat until state police have finished their own investigation into the crash.

“…[T]he limousine is a piece of evidence in a criminal investigation — it’s the court order the New York State Police has an obligation to follow, and they’re doing that,” she told the TV station.

Federal investigators expected they’d only have to wait a week or two after the Oct. 7 crash to get access to the vehicle, officials said nearly two months ago.

The vehicle failed to stop at an intersection and barreled into a Schoharie restaurant parking lot, killing two pedestrians and all 18 of the vehicle’s occupants — including four sisters who were later buried together.

The vehicle had failed a safety inspection the month before and was not supposed to be on the road.

Police arrested limo company operator Nauman Hussain and charged him with criminally negligent homicide.