ROME (AP) — Maltese Prime Minister Joseph Muscat says his chief of staff has resigned amid an ongoing investigation into the murder of investigative journalist Daphne Caruana Galizia.

Muscat has told reporters Keith Schembri informed him of his decision on Monday evening.

Schembri's resignation marks a key development in the probe Maltese investigators launched after Caruana Galizia's murder in a car bomb in October 2017.

Asked of the reasons behind Schembri's decision, Muscat said Tuesday it was premature to speculate on “whether he is being questioned or what he is being questioned about.” He added, however, that the timing of the resignation was “unfortunate.”

Schembri served as Muscat's chief of staff since 2013.

Caruana Galizia focused her investigations on corruption within the country's political and business ranks.