The leading cause of aviation deaths in 2015 was "unlawful interference," not technical failures, for the second year running, said Dutch safety consultancy firm To70, the Independent reported Sunday. With more than 900 airline deaths in the past two years, attacks have outweighed any other cause of plane accidents.

Unlawful interference on board by passengers is reasonably well-covered around the world," Adrian Young, an aviation consultant for To70, told the Independent. "My main concerns are centered on the way airport and airline staff get airside," he said, adding, "There are many airports that have weak systems to control who goes airside and with what.”

Malaysia Airlines flight 17 was shot down over eastern Ukraine — allegedly by pro-Russian separatists — in 2014, killing 283 people. In the same year, Malaysian Airlines flight 370 disappeared somewhere over the Indian Ocean with little evidence as to what occurred.

Developments in the MH370 case continued in July after a flaperon confirmed to be from the flight was found on Ile de la Reunion, a French island in the Indian Ocean. Investigators in the case have not yet determined what exactly caused the flight to disappear from radar in March 2014, and experts said they would need to find the black box, a data recorder, from the flight to understand what exactly had happened to the crew and its passengers.

The Islamic militant group known as ISIS took credit for blowing up a Russian passenger plane over Egypt’s Sinai Peninsula in November, killing all 224 people on board. The group said the attack was retribution for Russian airstrikes in Syria.

The second highest death toll for a plane crash in 2015 for “unlawful interference” was the Germanwings crash in the Alps in March 2015. The co-pilot of the plane allegedly purposefully crashed the plane, killing all 150 people on board.