The detentions came three days after members of the European Parliament completed a visit to Malta to examine its justice system. They voiced concern about a “culture of impunity” on the island, a problem that the Ms. Caruana Galizia had put at the center of her online blog, Running Commentary, and in a column in The Malta Independent newspaper.

Sven Giegold, a member of the European Parliament delegation, said in a statement on Friday that the group had arrived on the island “seriously concerned over the rule of law in Malta and left even more worried.”

“The police and the attorney general have demonstrated an unwillingness to investigate and failure to prosecute corruption and money laundering,” he added.

The delegation’s complaints added to momentum in Brussels for action against Malta under what is known as Article 7, a previously unused provision in European Union law that, in extreme cases, allows a country to be stripped of its voting rights and subjected to other punishments. That is unlikely to happen to Malta in the short term, but even the start of a full formal investigation by the European Commission, the bloc’s executive arm, would be a severe blow.

Michael Farrugia, Malta’s minister of home affairs, said on Twitter that the investigation had involved “foreign experts,” but he did not specify their nationalities. The F.B.I.; Europol, a European police organization; and officials from the Netherlands have been assisting Maltese investigators, but the island’s police commissioner has said that those agents were only providing technical support and were not involved in steering the investigation.

The death of Ms. Caruana Galizia, 53, prompted protests attended by thousands of people. She was killed as she drove near her home in a rented car. The blast was so powerful that the police took four days to collect body parts and wreckage.

At one rally, demonstrators marched through Valletta, the Maltese capital, singing patriotic songs and waving placards that quoted the last words known to have been written by Ms. Caruana Galizia: “There are crooks everywhere you look now. The situation is desperate.” Protesters accused the authorities of failing to protect her and of turning a blind eye to criminality inside and outside the government.