Maltese police believe Panama Papers journalist was killed with a bomb triggered by a text message

Police sources said investigators suspected George Degiorgio sent the text message after receiving a signal from his brother Alfred, who they believe acted as a lookout. A boat has been impounded. There was no immediate statement from the men's lawyers.

Evidence gathered so far suggests the bomb was placed inside Caruana Galizia's rented car while it was parked in an alley outside her house six miles from Valletta on the night before her death.

Mobile phones were recovered from the sea in Marsa, an inland area of Valletta harbour, the sources said.

Caruana Galizia wrote a popular blog which highlighted cases of alleged graft and targeted politicians in government, including Prime Minister Joseph Muscat, and the opposition.

Her blog sometimes got more than 400,000 readers a day - more than the combined circulation of all of Malta's newspapers.

Shortly before her death, she linked Muscat, and his wife, Michelle Muscat, to payments from the ruling family of Azerbaijan that were hidden in secret offshore bank accounts.

Thomson Reuters

Maltese investigative journalist Daphne Caruana Galizia poses outside the Libyan Embassy in Valletta

"In March last year, Al Sahra FZCO - a company incorporated in Dubai's free zone - made a single payment transaction of $1.017 million to Egrant Inc, a company incorporated in Panama in 2013," Galizia wrote in April.

The "daughter of Ilham Aliyev, ruler of Azerbaijan," was "the ultimate beneficial owner" of Al Sahra FZCO, Galizia added, and Michelle Muscat owns shares of Egrant Inc.

Al Sahra FZCO is an oil and gas company, and Azerbaijan's national oil company recently became a shareholder in Malta's new power plant.

Seven other men were arrested in connection with the probe and then released without charge, police said.

Caruana Galizia's family, which has criticised the handling of the case, said in a statement on Wednesday that the murdered journalist had not been investigating any of the 10 people arrested.

"The manner in which [yesterday's] arrests were communicated to Daphne Caruana Galizia's family indicates serious institutional deficiencies which are cause for general public concern," the statement also said. "The information about the arrests was communicated by the Prime Minister, and not by the Malta Police, who appear to prioritise informing the Prime Minister of developments to the exclusion of the surviving members of the assassination victim's family."

Simon Busuttil, former leader of the opposition in the Maltese parliament, tweeted a link to the statement, adding: "This can only mean that those who commissioned the assassination of #DaphneCaruanaGalizia are still out at large."

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Daphne Caruana Galizia and Matthew Caruana Galizia.

Caruana Galizia's son, Matthew Caruana Galizia, who is also a journalist and has blamed his mother's death on Muscat and the Maltese government, wrote on Facebook on Thursday that "whatever those politicians with a lot to lose say, the status of justice for my mother's murder remains the same: complete impunity."

Prime Minister Muscat has called the killing an attack on press freedom, and asked the US Federal Bureau of Investigation to help local police investigate.

Members of European Parliament who recently visited Malta on a fact-finding mission said they arrived "seriously concerned" about the rule of law on Malta and left "even more worried," according to The Guardian.

Ana Gomes, a Portugese MEP, said Maltese officials would not answer their questions, which the delegation found "extremely disturbing."