Daphne Caruana Galizia, a prominent investigative journalist in Malta, has been killed after a powerful bomb exploded in her car, police said, in an incident that stunned the small island country,

The 53-year-old had just driven away from her home in Mosta, a town outside Malta's capital of Valletta, when the bomb went off on Monday, sending the vehicle's wreckage spiralling over a wall.

The force of the blast reduced her car to pieces and catapulted the journalist's body into a nearby field, witnesses said. She leaves behind a husband and three sons.

Caruana Galizia ran a hugely popular blog in which she relentlessly highlighted cases of alleged high-level corruption by politicians from across party lines.

She had also exposed Malta's links with the so-called Panama Papers document leak, which revealed the identities of the rich and powerful around the world with offshore holdings in the Central American country.

"There are crooks everywhere you look now. The situation is desperate," Caruana Galizia wrote in a blog published on her site just half an hour before an explosion tore into her car.

Maltese Prime Minister Joseph Muscat, who faced accusations of wrong-doing by Caruana Galizia earlier this year, denounced her killing, calling it a "barbaric attack on press freedom".

He announced that the US FBI had agreed to help local police investigate the killing and was flying experts to the island as soon as possible.

"I will not rest until I see justice done in this case," he said in a statement, calling for national unity.

Opposition leader Adrian Delia called the killing a "political murder".

'Dark day for democracy'

Politico named Caruana Galizia as one of 28 Europeans who are "shaping, shaking and stirring" Europe.

Caruana Galizia had been sued for libel because of various articles she wrote on her Running Commentary blog, and she had filed a report with the police two weeks ago that she was receiving threats.

The journalist revealed that Muscat's wife, Michelle, as well as Muscat's energy minister and the government's chief-of-staff, held companies in Panama by looking into the 2016 document leak. Muscat and his wife deny they held such companies.

Monday evening's session of parliament was scrapped, except for briefings about the bombing scheduled to be given by Muscat and Delia, the opposition leader.

In June, Muscat was sworn in for a second term as prime minister following snap elections he had called to reinforce his government as the Panama Papers' leak indicated his wife owned an offshore company.

The couple denies wrongdoing.

The Committee to Protect Journalists (CPJ), a watchdog group, called on Maltese authorities to quickly ensure justice for Caruana Galizia.

"Daphne Caruana Galizia investigated wrongdoing in Malta's political, business and criminal worlds," said Robert Mahoney, CPJ deputy executive director. "The investigation into her murder therefore must be thorough, credible and timely."

European politicians and journalists expressed dismay at Caruana Galizia's death.

"A dark day for democracy," Manfred Weber, head of the conservative bloc in the European Parliament, wrote on Twitter.

Shocked & sickened by the death of Daphne Caruana Galizia. Freedom of expression is one of our most cherished values. It must be protected! — Pierre Moscovici (@pierremoscovici) October 16, 2017

Brutal murder of Daphne Caruana Galizia: tragic example of journalist who sacrificed her life to seek out the truth. She won't be forgotten. — EP President Tajani (@EP_President) October 16, 2017