Federal safety leaders are uncovering new details about a tragic mid-air collision in Washington, D.C., that claimed 67 lives, with hearings revealing potential issues with helicopter altimeters.

The federal agency will hold a series of hearings to hear from board members, investigative staff, and witnesses to the January 29 collision that killed two pilots, two flight attendants, and 60 passengers aboard Flight 5342, along with the helicopter’s three crew members.