MILAN (AP) — Maltese authorities faced mounting pressure Thursday to ensure an independent investigation into the slaying of investigative journalist Daphne Caruana Galizia, who was known for her fearless reporting on corruption in the small Mediterranean island nation.

The 53-year-old journalist's three grown sons called on Maltese Prime Minister Joseph Muscat to resign, saying he should take political responsibility for "failing to uphold our fundamental freedoms" by not rooting out corruption.

A group of U.N. human rights experts also demanded that the government of Malta "honor its commitment to a prompt, independent investigation" into the journalist's assassination Monday in a powerful car bomb.

Muscat, in Brussels for a European Union leaders' summit, did not respond directly to the family's call for his resignation, but he denied their allegations that Malta is a mafia state.

"Definitely not. I don't want to be rushed into commenting on the comments and statements of a son who is in mourning," Muscat told reporters. "If I had found my mother butchered in a bomb attack, probably I would have said worse."

Muscat has denounced the assassination and has proposed a reward to find her killers. He pledged to leave "no stone unturned" in the investigation, which is being assisted by the FBI, Scotland Yard, Europol and Dutch forensic experts.

At a press conference, Police Commissioner Lawrence Cutajar said collecting evidence from the scene could take weeks. He said FBI and Dutch experts were brought in "because we felt we could have difficulties lifting evidence."

"The situation is very fragile and we cannot reveal details," he said. "My main focus is to solve this crime," which he described as "macabre."

Investigators are still working to determine the type of explosive and how the bomb was triggered.

Caruana Galizia was one of Muscat's harshest critics, revealing connections by his wife and members of his government to shell companies in Panama. The Muscats have denied the allegations.

Caruana Galizia was also strong in her criticism of Malta's opposition politicians.

In a Facebook post, her sons Matthew, Andrew and Paul Caruana Galizia said they weren't endorsing Muscat's call for a reward, saying "we are not interested in justice without change."

"We are not interested in a criminal conviction, only for the people in government who stood to gain from our mother's murder to turn around and say that justice has been served," they said.

Her sons wrote that identifying their mother's assassins was not enough — they said corruption on the island nation also needed to be rooted out.

People hold flowers and candles during a vigil outside of EU headquarters in Brussels. (AP Photo/Virginia Mayo)

The U.N. experts said that the family's concerns about the independence of the investigation "should be taken seriously."

"We are pleased that the Maltese authorities have initiated an investigation into the murder," they said. "We now urge a prompt, thorough and independent public inquiry and investigation, followed by a full judicial process to hold all the perpetrators to account."

On Thursday, some 200 journalists rallied in support of the slain journalist.

The Institute of Maltese Journalists has filed a court case seeking to ensure source confidentiality on all data that is lifted from Caruana Galizia's computers and mobile phones during the homicide investigation.

Investigators, meanwhile, were looking at similarities with other car bombings in Malta over the last two years — six in all, including Caruana Galizia's. None have been solved.

Former police commissioner John Rizzo told the Malta Independent it appears that mobile-detonated explosives were used in each of the six bombings since the start of 2016, which caused four deaths and two serious injuries. The previous victims were all known to police, the paper said.

Caruana Galizia's car. (AP Photo/Rene Rossignaud)

"Very few people could construct such a bomb. Instructions may be obtained online but building such a device would still require a certain degree of skill," Rizzo said.

While the type of explosive remains unknown, experts say military grade explosives like Semtex are not available in Malta and would have had to be brought in from abroad.

Muscat defended his government's failure to solve the rash of car bombings. In all, there have been over 30 in the last 15 years on the island.

"I will continue to defend the institutions," he said.

___

Lorne Cook contributed from Brussels.