Slain Maltese journalist Daphne Caruana Galizia was a determined investigative reporter fuelled by outrage at the cronyism and sleaze she saw engulfing her island nation.

Her assassination in a car bomb on October 16, 2017 at the age of 53 ended 30 years of reporting on scandals, from petrol smuggling to money laundering, that implicated members of the government and organised crime.

On Wednesday, Malta police arrested businessman Yorgen Fenech, with a police source identifying him as "a person of interest" in Caruana Galizia's death.

Three men in custody since late 2017 have been charged with her murder, but the mastermind of the crime has never been identified.

A former reporter for the local Sunday Times and the Malta Independent, Caruana Galizia was best known as the publisher, from 2008 onwards, of her widely read blog, Running Commentary.

Mixing political exposes with acerbic commentary, the blog was required reading for anyone interested in Malta's byzantine and highly polarised politics.

After Caruana Galizia exposed what the massive Panama Papers data leak revealed about government corruption in her country, news site Politico described her as "a one-woman WikiLeaks, crusading against untransparency and corruption in Malta, an island nation famous for both".

Caruana Galizia was regularly hit or threatened with defamation suits, including by Prime Minister Joseph Muscat. That attention helped swell her readership to levels estimated to be higher than all of Malta's mainstream media combined.

At the receiving end of intimidation and threats, Caruana Galizia received police protection several times but refused to be put under permanent guard, judging it would make her job impossible.

The blog written by journalist Daphne Caruana Galizia was required reading for anyone interested in Malta's byzantine and highly polarised politics
The blog written by journalist Daphne Caruana Galizia was required reading for anyone interested in Malta's byzantine and highly polarised politics AFP / Matthew Mirabelli

Caruana Galizia was survived by her lawyer husband, Peter, and three sons, Matthew, Andrew and Paul.

Since her death, son Matthew has maintained that justice has not been done. Malta's police, he said, were unwilling to take on the "highest level of organised crime that involve government".

"Three people have been arrested, but they are the hitmen, they are right at the bottom. We don't know who sent them, who paid them," he told AFP in October 2018. "There isn't any justice whatsoever yet."

The three men - brothers Alfred Degiorgio and George Degiorgio, and Vince Muscat - were arrested in December 2017 and charged with the murder. They have pleaded not guilty.

Last week, Maltese police and Interpol arrested several people as part of an anti-money laundering operation, including the alleged middleman in Caruana Galizia's killing, who remains unnamed.

On Tuesday, Prime Minister Joseph Muscat said he had promised that individual a pardon were he to identify the mastermind of the assassination.

Before her death, Caruana Galizia expressed concerns over her safety and was considering leaving Malta.

In the final entry on her blog, posted barely half an hour before she was killed, Caruana Galizia signed off with a weary observation that: "There are crooks everywhere you look now. The situation is desperate."