Federal safety investigators have been unable to conduct a full examination of the limousine involved in a crash that killed 20 people nearly two weeks ago in upstate New York because local prosecutors are probing it as part of their case against the limo company's operator.

While a National Transportation Safety Board spokesman says it is working cooperatively with local officials, people familiar with the matter told The Associated Press on Thursday that investigators have privately expressed frustration over their inability to fully examine the limousine.

They spoke to the AP on condition of anonymity in order to discuss sensitive conversations.

The limo remains in the possession of New York State Police after the limousine company's operator was charged four days after the crash with criminally negligent homicide.

A state police spokesman said it could be several more weeks before the NTSB is granted hands-on access to the limo. The NTSB would get in line behind state investigators and the lawyer for the limo company's operator.

"The vehicle is the most important piece of evidence that will help ultimately determine the cause of the crash, and the extent of any criminal wrongdoing," spokesman Beau Duffy said in a statement. "If the NTSB were allowed to handle evidence before it has been fully examined and processed by the state police and the defense, it would jeopardize the criminal case."

NTSB spokesman Eric Weiss said investigators were able to look inside the limousine briefly and have not conducted a full examination. But he stressed that the agency is working closely with state police.

"We anticipate getting everything we need in a timely fashion," Weiss said. "They have a criminal investigation to do. We have to accommodate that."

The federal agency is charged by Congress to conduct independent probes and can make urgent safety recommendations to address specific issues discovered during an investigation. The NTSB expects to release a preliminary report on the wreck in the next several weeks, Weiss said.

The district attorney in Schoharie County did not immediately return a call from the AP seeking comment on Thursday.

The limousine loaded with 18 people on their way to a birthday party for one of the occupants ran a stop sign and crashed at the bottom of a hill in the town of Schoharie. Everyone in the limo died, including four sisters, along with two pedestrians.

Prosecutors allege the limousine company's operator, Nauman Hussain, allowed an improperly licensed driver to operate an "unserviceable" vehicle. He has pleaded not guilty to the charge and has declined to comment on the crash.

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Klepper reported from Albany, N.Y. Associated Press writer Mary Esch in Albany, N.Y., contributed to this report.