**2. Methods**

The U.S. National Transportation Safety Board (NTSB) database was searched on 26 June 2018, using the following search words: "suicide", "murder–suicide", and "homicide–suicide" [26]. Fatal aviation accidents in the U.S, with full formal accident investigation reports finalized at the time of the search and the cause of accidents assessed as pilot suicide in the NTSB accident investigation, were included as index cases to this study. These NTSB accident investigations were further analyzed

in-depth to assess the descriptive factors in these suicide processes. Only those accidents in which NTSB judged that the cause of the accident was suicide were included in the statistical analysis.

Frequency of the aircraft-assisted pilot suicides out of all fatal accidents in the U.S was calculated for the reference period (5 years before the 11 September 2001 terrorist murder-suicides event) and separately for the first, second, and third year after the September 11 terrorist attacks. The relative risk (RR) of the likelihood of an aircraft accident being due to pilot suicide for the three post-attack years was calculated.
