A top aide to Malta's Prime Minister has resigned following revelations that he was questioned by police investigating the murder of a prominent journalist.

Key points: Daphne Caruana Galizia investigated corruption in the Maltese Government

Daphne Caruana Galizia investigated corruption in the Maltese Government Businessman Yorgen Fenech was arrested this week but not charged

Businessman Yorgen Fenech was arrested this week but not charged Ms Caruana Galizia's son has urged the Prime Minister to resign

Two Government ministers have also stood aside in the widening scandal.

Prime Minister Joseph Muscat said his chief of staff Keith Schembri had quit as police stepped up their investigation into the 2017 killing of Daphne Caruana Galizia, an investigative reporter who repeatedly exposed corruption at the highest levels of government.

Tourism Minister Konrad Mizzi also announced his resignation and Economy Minister Christian Cardona said he was suspending himself from his duties until the investigation concluded.

Both Mr Mizzi and Mr Cardona denied any involvement in the death of Ms Caruana Galizia, who was killed in a car bomb.

Police searched Mr Schembri's house as part of their investigation, sources said, but gave no further details.

Neither Mr Schembri nor his lawyer were immediately available for comment.

Mr Cardona was briefly questioned by police on Saturday.

Daphne Caruana Galizia was described as a "one-woman WikiLeaks". ( AP: The Malta Independent )

Mr Schembri and Mr Mizzi had been facing pressure from opposition politicians and Ms Caruana Galizia's family to resign over their purported financial ties to businessman Yorgen Fenech.

Mr Fenech was arrested last week and later released by police without being charged. He will be subjected to around-the-clock police surveillance.

Eight months before she was killed, Ms Caruana Galizia alleged that a company named 17 Black Ltd., which was listed in the Panama Papers, was connected to Maltese politicians. The company belonged to Mr Fenech.

Presidential pardon for 'middleman'

The suspected middleman in the murder, Melvin Theuma, was granted a presidential pardon on Monday in return for evidence that could be used in court. Media on the Mediterranean island reported that he had handed over audio recordings.

"What is happening now, and what happened last week, is an operation that is hopefully solving the Daphne Caruana Galizia murder," Mr Muscat said.

"Whatever people might say, there is no impunity in this country."

Mr Muscat is a close friend of Mr Schembri and had repeatedly rejected calls to sack him when allegations of corruption surfaced several years ago. Mr Schembri denied wrongdoing.

'Bring the house down'

According to a person briefed on the investigation, Mr Theuma started to provide further information that would "bring the house down", with substantial details of corruption and the complicity of other senior figures in the murder that shocked Europe.

The source said Mr Fenech was insisting on being given immunity from prosecution. That requires the approval of Malta's President, on the advice of his Prime Minister and cabinet.

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Three men are awaiting trial for setting off the bomb that killed Ms Caruana Galizia, and police sources said the investigation into who ordered the killing had made significant advances in the past week.

Opposition politicians and Ms Caruana Galizia's family have denounced Mr Muscat for allowing the two men to stay in their posts and have called on him to step down.

A regular session of Parliament was suspended on Tuesday as opposition politicians heckled the Government benches, yelling "mafia" at their political opponents.

Mr Muscat said he had no intention of resigning.

"My role right now is to ensure the country has stable leadership. My role is to make sure we navigate through this turbulent time," he said

Ms Caruana Galizia often targeted members of the Muscat Government in her blogs, including Mr Schembri and Mr Mizzi. Six months before her death, the two men said they would sue her for libel over allegations she made about their Panama concerns.

Reuters/AP