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Maltese Justice Minister Owen Bonnici speaks to the media outside Parliament, in Valletta, Malta, on Dec. 2, 2019.VINCENT KESSLER/Reuters

The European Commission urged Malta on Monday to speed up reforms to ensure the independence of its judiciary amid uproar over the investigation into the 2017 murder of journalist Daphne Caruana Galizia.

In a letter to Maltese Justice Minister Owen Bonnici, European Justice Commissioner Didier Reynders said Malta should carry out a thorough investigation, free from political interference, to bring those responsible to justice.

A drawn-out inquiry has led Prime Minister Joseph Muscat to offer his resignation after his former chief of staff was briefly arrested by police investigating the murder.

Malta, Mr. Reynders said, needed to ensure the independence of judicial appointments and dismissals and a separate prosecution service, as requested by the European Union in July.

“Recent controversies have underlined that progress should accelerate, for example, on securing an effective and autonomous prosecution service with clear protections from the risk of political interference,” Mr. Reynders wrote.

Mr. Muscat has said he will resign in mid-January once a new leader of his Labour Party is picked, but the European Parliament told EU leaders last Thursday he should go immediately to ensure the investigation is not compromised.

Caruana Galizia’s family say they suspect Mr. Muscat’s government of a cover-up. Mr. Muscat denies wrongdoing.

“The continued concerns surrounding the investigation have put the spotlight on a number of more systemic issues,” Mr. Reynders wrote, adding that all EU countries should respect the rule of law.

Mr. Reynders advised Malta to set out a clear reforms timetable and to consult the Venice Commission, an advisory body of legal experts for the Council of Europe, on all draft legislation.

Two years after the car bomb that killed the anti-corruption journalist, police have been unable to say who hired the killers. Last month, a man given immunity from prosecution confessed to being the middleman.

Since then, one of Malta’s wealthiest men has been charged with complicity to murder. Mr. Muscat’s government has denied his request for immunity in return for testimony that he said would implicate senior officials.

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