The National Transportation Safety Board said Tuesday it will investigate a December flight in which a United Airlines plane descended to within less than 800 feet (250 meters) of the ocean surface after taking off from Hawaii.
The NTSB said it expects to issue a preliminary report in two to three weeks.
The agency had told The Associated Press on Monday that it was asking United questions about the incident before deciding whether to launch a formal investigation.
The Boeing 777 dropped more than 1,400 feet (470 meters) before regaining altitude and completing the Dec. 18 flight from Kahului Airport on the island of Maui to San Francisco, according to data from tracking service Flightradar24. No injuries were reported.
Chicago-based United said it is cooperating with authorities and the pilots are currently receiving additional training.
