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Daphne Caruana Galizia died on Monday when her car, a Peugeot 108, was destroyed by a powerful explosive device, reports the Guardian.
A blogger whose posts often attracted more readers than the combined circulation of the country's newspapers, Caruana Galizia was recently described by the American news outlet Politico as a "one-woman WikiLeaks".
Her latest revelations accused Malta's Prime Minister Joseph Muscat and two of his closest aides, connecting offshore companies linked to the three men with the sale of Maltese passports and payments from the government of Azerbaijan.
No group or individual claimed responsibility for the attack, the Guardian reported.
Malta's President Marie-Louise Coleiro Preca, called for calm. "In these moments, when the country is shocked by such a vicious attack, I call on everyone to measure their words, to not pass judgement and to show solidarity."
In a statement, Muscat condemned the "barbaric attack".
"Everyone knows Caruana Galizia was a harsh critic of mine," said Muscat, adding "Both politically and personally, but nobody can justify this barbaric act in any way."
He announced in parliament later on Monday that Federal Bureau of Investigation (FBI) officers were on their way to Malta to assist with the investigation, following his request for help from the US government.
According to local media reports, Caruana Galizia filed a police report 15 days ago to say that she had been receiving death threats.
The journalist posted her final blog on her Running Commentary website at 2.35 p.m. on Monday, and the explosion, which occurred near her home, was reported to police just after 3 p.m.
Over the last two years, her reporting had largely focused on revelations from the Panama Papers, a cache of 11.5 million documents leaked from the internal database of the world's fourth largest offshore law firm, Mossack Fonseca.