A top aide to Malta's prime minister resigned Tuesday and was questioned by police investigating the murder of a prominent journalist, while two government ministers also stood aside in a widening scandal rocking the European Union's smallest state.

Prime Minister Joseph Muscat told reporters 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 denounced 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 murder investigation was concluded.

Both Mizzi and Cardona denied any involvement in the death of Caruana Galizia, who was blown up by a car bomb.

Police searched Schembri's house during the day, sources said, but gave no further details. Neither Schembri nor his lawyer was immediately available for comment.

Schembri and Mizzi had been facing pressure to resign from opposition politicians and Caruana Galizia's family because of their purported financial ties to businessman Yorgen Fenech, who was detained last week as a person of interest.

Cardona was briefly questioned by police on Saturday.

Malta Trade Minister Christian Cardona arrives at a EU Competitiveness Council meeting at the European Council, in Brussels in May 2017. He was suspending himself from his duties until the murder investigation was concluded. (Stephanie Lecocq/EPA)

The suspected middleman in the murder, Melvin Theuma, was granted a presidential pardon 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," Muscat told Reuters on Tuesday. "Whatever people might say, there is no impunity in this country."

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

'Bring the house down'

Police had planned to charge Fenech on Tuesday on murder charges before a magistrate.

But, according to a person briefed on the investigation, he started to provide further information which would "bring the house down," with substantial details of corruption and the complicity of other senior figures in the murder.

Fenech, said the same person, is insisting that like Theuma, he too is given immunity from prosecution. That requires the approval of Malta's president, on the advice of his prime minister and cabinet.

Fenech was released Tuesday on police bail as they seek to check whether the new information is true.

Maltese businessman Yorgen Fenech, wearing dark sunglasses, is accompanied by police during a search of his yacht at the dock of Portomaso, Malta, on Friday. (Associated Press)

Three men are awaiting trial for setting off the bomb that killed Caruana Galizia, and police sources say the investigation into who actually ordered the killing has made big advances in the past week.

Neither Fenech nor his lawyer has made any comment since his detention.

A Reuters investigation last year revealed that Fenech was the owner of a secret company called 17 Black, which was named in emails as being the vehicle to fund secret Panama companies owned by Schembri and Mizzi.

It is not known whether any funds actually changed hands. Schembri, who has many business interests, and Mizzi have always denied wrongdoing.

Maltese Prime Minister's chief of staff Keith Schembri is seen in Valletta, Malta, in May 2018. (Darrin Zammit Lupi/Reuters)

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

"Muscat, who spent three years taking full responsibility for Schembri, must leave right now. He simply cannot be trusted to not obstruct justice. He has done so for too long," Caruana Galizia's son Matthew wrote on Twitter.

A regular session of parliament was suspended on Tuesday as opposition lawmakers heckled the government benches, yelling "mafia" at their political opponents.

Malta's Tourism Minister Konrad Mizzi addresses a tourism conference at the Hilton Hotel in St Julian's, Malta, in September 2018. Mizzi has announced his resignation. (Darrin Zammit Lupi/Reuters)

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 told Reuters.

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