A key piece of hardware that aviation experts and pilots said could solve the mystery in the fatal crash of the medical transport Learjet on Jan. 31 turned out to be useless. The cockpit voice recorder recovered 8-feet deep at the bottom of the jet’s impact crater was repaired after extensive damage,

The cockpit voice recorder was not working on a medical transport plane that crashed in Philadelphia in January and likely had not been working for several years, the National Transportation Safety Board said in a preliminary report issued Thursday.

Investigators have not yet determined what caused the Learjet 55 to nosedive seconds after takeoff on Jan. 31. The crash killed 7 people and injured 24 others.