NTSB investigators have recovered the engines and cockpit voice recorder (CVR) of the Learjet 55 that crashed on Jan. 31 in Philadelphia, killing at least seven people.

The so-called “black box” is one of the most important pieces of forensic evidence following a plane crash. There are actually two of these remarkably sturdy devices.

A black box from the medical jet that crashed in Northeast Philadelphia has been found, the NTSB said on Sunday.