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 National Transportation Safety Board is looking into what may have caused the deadly midair collision over the Potomac between a Black Hawk helicopter and a regional jet. The NTSB says that data from the chopper’s black box shows that it was flying too high but the pilots may not have known it because of potential altimeter malfunctions.