"There are crooks everywhere you look now. The situation is desperate," she wrote in a blog published on her site just half an hour before an explosion tore into her car.
Locals said Caruana Galizia had just left her house and was on a road near the village of Bidnija in northern Malta when the bomb detonated, sending her car flying into an adjacent field.
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 U.S. Federal Bureau of Investigation (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 said the blogger was the victim of a "political murder".
"Caruana Galizia revealed the Panama Papers and was the government's strongest critic," he said, calling for a independent probe of her killing.
"We will not accept an investigation by the Commissioner of Police, the Army commander or the duty magistrate, all of whom were at the heart of criticism by Caruana Galizia," he said.
WikiLeaks founder Julian Assange said he would offer a 20,000 euro ($23,578.00) reward for information leading to the conviction of Caruana Galizia's killers, and European politicians expressed dismay at her death.
Frans Timmermans, first vice president of the European Commission, tweeted that he was "shocked and outraged", adding that "if journalists are silenced our freedom is lost".
Manfred Weber, head of the conservative bloc in the European Parliament, said the killing marked "a dark day for democracy".
Caruana Galizia took aim at politicians and senior officials from across Malta, seeing the island as a hotbed of corruption.
"Malta's public life is afflicted with dangerously unstable men with no principles or scruples," she wrote last year.
Her family asked that the magistrate assigned to investigate the case, Consuelo Scerri Herrera, be substituted because of an alleged conflict of interest, court documents showed. Herrera had sought libel damages after Caruana Galizia attacked her in her blog.
(Additional reporting by Alastair Macdonald in Brussels and Gavin Jones in Rome,; Writing by Crispian Balmer; Editing by Catherine Evans)