Investigative reporter Daphne Caruana Galizia – dubbed a “one-woman WikiLeaks” – was killed as she was driving near the village of Bidnija in northern Malta.
She had filed a complaint to police a fortnight ago after receiving personal threats to her safety, local media said.
Caruana Galizia ran a hugely popular blog relentlessly highlighting cases of alleged corruption, often involving politicians from the Mediterranean island nation.
In Malta, she had led the coverage of the Panama Papers, a leak of millions of legal documents last year detailing financial details of offshore entities used by the rich and powerful to avoid tax.
Video today showed the burnt-out wreck of her car lying in a field metres from the road.
Maltese Prime Minister Joseph Muscat, who faced accusations of wrong-doing by Caruana Galizia earlier this year, denounced her killing.
He called it a “barbaric attack on press freedom”.
“Our country deserves justice,” he said in a televised statement, calling for national unity.
“I will not rest until I see justice done in this case.
“Everyone knows Caruana Galizia was a harsh critic of mine, both politically and personally, but nobody can justify this barbaric act in any way.
“The only remedy for anyone who felt slandered was through the courts.”
Muscat had called early elections in June seeking a vote of confidence to counter Caruana Galizia’s allegations of corruption.
The report had said documents in a small Malta-based bank showed that Muscat’s wife was the beneficial owner of a company in Panama, and that large sums of money had been moved between the company and bank accounts in Azerbaijan.
Both Muscat and his wife denied the accusation and sued Caruana Galizia for defamation.
Muscat easily won reelection.
Recently, Caruana Galizia’s outspoken blog had turned its fire on opposition politicians.
On Monday, she had written: “There are crooks everywhere you look now. The situation is desperate.”
In another entry last year, she wrote: “Malta’s public life is afflicted with dangerously unstable men with no principles or scruples.”