VALLETTA (Reuters) - The son of Malta s best-known investigative journalist said on Tuesday his mother had been killed by a car bomb because of her work exposing political corruption, as hundreds of people demonstrated to demand justice after her death. Daphne Caruana Galizia, who wrote about graft across Malta s political divides on her blog, died when explosives ripped through her car minutes after she left her home in the north of the island on Monday afternoon. Maltese authorities were waiting for the arrival of Dutch forensic experts and American FBI agents to help the investigations.  My mother was assassinated because she stood between the rule of law and those who sought to violate it, like many strong journalists,  Matthew Caruana Galizia said on Facebook.  She was also targeted because she was the only person doing so,  he added. He described rushing to the scene, only to find the burning car and her remains. Maltese Prime Minister Joseph Muscat, who was accused of wrong-doing by Caruana Galizia earlier this year and had been suing her over some of her allegations, denounced her killing and pledged to track down those responsible. But a small group booed Muscat on Tuesday evening when he left his office, and the opposition is calling for him to resign.  The prime minister and his government have been responsible for diminishing the rule of law in Malta,  opposition leader Adrian Delia told Reuters.  They have created an environment where people are afraid to speak out.  On Tuesday afternoon, several hundred people demonstrated in front of the law courts demanding justice for Caruana Galizia s killers.  The state did not defend Daphne,  shouted Andrew Borg Cardona, addressing the crowd. He said those who accused her of  going over the top  with her investigations  are all guilty . One woman carried a votive lamp with the murdered journalist s picture in it and another carried a sign that read  Looks like we can t have freedom of speech but we want justice . Recently, Caruana Galizia had been following up leads from information in the so-called Panama Papers, a large collection of documents from an offshore law firm in the Central American nation that were leaked in 2015. She was tracing alleged links between Maltese officials and offshore banks and companies used as tax havens. Half an hour before the explosion, Caruana Galizia wrote on her blog:  There are crooks everywhere you look now. The situation is desperate.  The European Commission told journalists it was horrified by the murder in the bloc s smallest state and called for justice. Spokesman Margaritis Schinas was asked if the Commission would open a procedure to check if Malta was meeting the EU s standards for the rule of law, a process now being applied to Poland over judicial reforms there. He replied:  We never speculate on these questions. These are very serious subjects ... This is an outrageous act that happened, and what matters now is that justice will be brought.   This is what we need to see.  The killing near the village of Bidnija stunned the Mediterranean island. Authorities said it was the first murder of a journalist there.  I saw a small explosion coming from the car and I panicked. A few seconds later, around three to four seconds, there was another, larger explosion,  said resident Frans Sant, who was driving in the other direction.  The car continued coming down the hill, skidding at high speed, full of fire. The car missed me by around 10 feet. I tried to help, but the fire was too much and the car ended up in the field,  he told Reuters Television. 