The Learjet's owner before Neil hired a mechanic who performed a hard-landing inspection. Investigators interviewed the mechanic who said he used a maintenance manual for all the work he performed.

PHILADELPHIA — 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.

Credit: NTSB The cockpit voice recorder did not record the moments before a Learjet 55 medical jet crashed earlier this year in a Philadelphia neighborhood, killing six people on the aircraft and ...