ALBANY, N.Y. (CBSNewYork) – The NTSB says it has not been able to fully investigate the deadly limo crash in upstate New York that killed 20 people in October.

Federal investigators blame the Schoharie County district attorney’s office, saying the NTSB has been denied full access to the limo.

County district attorney Susan Mallery told CBS station WRGB-TV in Albany that the limo is evidence in a criminally negligent homicide case against its owners.

She also says her office is not hindering NTSB access to the limo.

READ: Remembering Limo Crash Victims: ‘They Would Do Anything For You’

A group of 18 passengers in the limousine died after the vehicle barreled through a stop sign and crashed west of Albany on Oct. 6. Two pedestrians also died.