In April 2007, the NTSB made four further recommendations, three measures to avoid fatigue affecting the performance of air traffic controllers, and one to prevent controllers from carrying out non-essential administrative tasks while aircraft are taxiing under their control. Although these recommendations were published during the course of the NTSB's investigation into the accident to Comair Flight 5191, they were in part prompted by four earlier accidents, and the board was unable to determine whether fatigue contributed to the Comair accident.