Mechanical and design problems with a Boeing 737 MAX flight control system were key factors in the crash of a Lion Air jet last year, Indonesian investigators told victims’ families in a briefing on their findings Wednesday.

But some grieving relatives expressed anger that investigators were not planning to blame the US airplane maker or budget carrier for the disaster in their final crash report expected Friday.

“We’re not satisfied with the explanation from investigators, but we have no choice but to accept it,” Epi Syamsul Qomar, who lost his son, told reporters after a closed-door meeting with investigators in Jakarta.

All 189 passengers and crew were killed when the Boeing 737 MAX slammed into the Java Sea shortly after takeoff from Jakarta on October 29 last year.

In a slide-show presentation, Indonesia’s National Transportation Safety Committee told relatives their report would include problems with the MCAS as a “contributing factor”.

“During the design and certification of the , assumptions were made about pilot response to malfunctions which, even though consistent with current industry guidelines, turned out to be incorrect,” said the presentation, part of which was provided to AFP.

It added that the system was vulnerable to the sole sensor it relied on for inputs, and that a replaced sensor on the doomed Lion Air plane had been “miscalibrated” during an earlier repair.An “absence of guidance” to 737 MAX pilots added to the challenges of dealing with a malfunction, according to the presentation.