Officials believe a bomb that killed an investigative journalist in Malta was attached beneath her car and triggered remotely.
A government spokeswoman said this assumption was based on initial findings from a police investigation into the death of the journalist.
Daphne Caruana Galizia, who was known for exposing government corruption, died on Monday in a blast that destroyed her car as she was leaving her house.
One of the journalist's sons, Matthew Caruana Galizia, on Tuesday used a Facebook post to describe running toward his mother's car after hearing the explosion, only to see "my mother's body parts all around me."
The murder shocked Malta, the smallest nation in the European Union, and Prime Minister Joseph Muscat on Wednesday promised a reward to anyone who came forward with information about the killing.
Ms Galizia's three sons dismissed the offer, and called instead for Mr Muscat to resign, saying: "People who for as long as we can remember sought to silence our mother cannot now be the ones to deliver justice."
Mr Muscat has ruled out quitting and flew to Brussels on Thursday for an EU summit, where his spokeswoman said investigators were beginning to make progress.
"Emerging evidences make us think that the bomb was placed under the car and was set off with a remote trigger," she said, adding that foreign experts would be called on to help identify the mobile phone which was used to detonate the bomb.
In a news conference in Valletta, police commissioner Lawrence Cutajar denied British police would join Dutch forensic experts and a team from the US Federal Bureau of Investigation (FBI) in helping with the case. Mr Muscat's spokeswoman earlier said British officers would be involved.
Mr Cutajar said no arrests had made so far and added it was too soon to discuss possible motives, saying it would take weeks to collect all the evidence. He said he could not confirm reports from a Maltese police source that Semtex was used in the killing.
The Mediterranean island has seen a number of small bomb attacks in recent years tied to gangland criminals, but the explosives used in those attacks were relatively crude and did not have the same power as the device that targeted Ms Caruana Galizia.
The 53-year-old journalist used her blog to launch scathing attacks on Mr Muscat, his wife and some of his closest advisers, accusing them of setting up offshore accounts to conceal corrupt money.
They denied the accusations and Mr Muscat was suing Ms Caruana Galizia for libel at the time of her death.
"The police may or may not find out who ordered the assassination of our mother but as long as those who led the country to this point remain in place, none of it will matter," her three sons, Matthew, Andrew and Paul, wrote on Facebook.
That added that the only way forward was for Mr Muscat to stand aside: "Resign for watching over the birth of a society dominated by fear, mistrust, crime and corruption."
As he arrived at the EU summit in Brussels, Mr Muscat denied that he had created a "mafia state" in Malta, which is home to a large financial services sector and the continental hub for the flourishing online gaming industry.
"Definitely not," Mr Muscat said.
The European Parliament said it would hold a debate next week on the protection of journalists and media freedom in Malta, where the government enjoys sweeping powers over the judiciary and the police.
"Malta is a Mecca for money launderers and tax avoiders," Greens EU legislator Sven Giegold said.
The prime minister says the financial services sector is as transparent and compliant as in any other European jurisdiction.
Reuters contributed to this report
