I believe several posters are getting confused about the event. United never disclosed the event to the NTSB or FAA. The flight crew voluntarily disclosed the event to United. United investigated the event internally and closed the event out with extra training for the crew. They did not apparently retain pertinent information like the flight data recordings.
Two months after the event someone whispered in the NTSB’s ear. They initiated a investigation at that time. The incident was not a mandatory required report by UAL.

Aircraft Accident and Incident Reporting

1. Occurrences Requiring Notification. The operator of an aircraft must immediately, and by the most expeditious means available, notify the nearest National Transportation Safety Board (NTSB) Field Office when:
   1. An aircraft accident or any of the following listed incidents occur:
      1. Flight control system malfunction or failure.
      2. Inability of any required flight crew member to perform their normal flight duties as a result of injury or illness.
      3. Failure of structural components of a turbine engine excluding compressor and turbine blades and vanes.
      4. Inflight fire.
      5. Aircraft collide in flight.
      6. Damage to property, other than the aircraft, estimated to exceed $25,000 for repair (including materials and labor) or fair market value in the event of total loss, whichever is less.
      7. For large multi‐engine aircraft (more than 12,500 pounds maximum certificated takeoff weight):
         1. Inflight failure of electrical systems which requires the sustained use of an emergency bus powered by a back‐up source such as a battery, auxiliary power unit, or air‐driven generator to retain flight control or essential instruments;
         2. Inflight failure of hydraulic systems that results in sustained reliance on the sole remaining hydraulic or mechanical system for movement of flight control surfaces;
         3. Sustained loss of the power or thrust produced by two or more engines; and
         4. An evacuation of aircraft in which an emergency egress system is utilized.
   2. An aircraft is overdue and is believed to have been involved in an accident.