The head of a European Parliament mission to Malta on Wednesday voiced "serious concern" about Joseph Muscat remaining prime minister during a probe into journalist Daphne Caruana Galizia's murder. "We have serious concern about Mr Muscat's stay in office," Dutch MEP Sophie in't Veld told journalists despite Muscat saying he would resign after January 12 amid a scandal-dogged probe into the 2017 killing. "We have concern about the integrity of the murder investigation," said the head of the seven MEP mission to the Mediterranean island. Caruana Galizia, a mother of three described as a "one-woman WikiLeaks", was blown up in a car bomb attack near her home in October 2017. She was known for exposing cronyism and sleaze within the country's political and business elite. Three men are facing trial for carrying out the assassination, but who ordered the murder remains a mystery. A rising tide of mass protests have called for Muscat to step down immediately over what critics have called a botched investigation into the car bombing amid wide-ranging corruption allegations. "We are very concerned with what we have seen or heard in the last two days," said In't Veld. The outgoing prime minister has been accused of obstructing justice by protecting political allies and aides. Last week his chief of staff, Keith Schembri, and tourism minister Konrad Mizzi resigned. Caruana Galizia had alleged that Schembri and Mizzi were involved in corruption, claims both men have denied. Tycoon Yorgen Fenech, arrested as he tried to leave on his yacht, has been charged with complicity in the murder. Fenech has claimed Schembri was the "real mastermind" behind the killing, according to police sources. Schembri was arrested last week but released two days later, fuelling accusations of a cover-up. "We don't want any risk or even any perceived risk that the investigation may be compromised in any way," In't Veld said. "There must be absolute confidence in the process and I think when he (Muscat) is in office that confidence is not there." Caruana Galizia's family has long pushed for Muscat's resignation, alleging that he has protected those around him.