VALLETTA (Reuters) - The bomb used to kill Maltese anti-corruption blogger Daphne Caruana Galizia was probably triggered by a call from a boat off Malta, an investigator said on Tuesday, laying out initial evidence against three suspects.

FILE PHOTO: Forensic experts walk in a field after a powerful bomb blew up a car (Foreground) and killed investigative journalist Daphne Caruana Galizia in Bidnija, Malta, October 16, 2017. REUTERS/Darrin Zammit Lupi

Caruana Galizia died in the powerful blast as she was driving near her home in the village of Bidnija on Oct. 16 -- a killing that appalled Europe and raised questions about the rule of law on the tiny Mediterranean island.

Brothers Alfred and George Degiorgio, and Vince Muscat were arrested earlier this month in connection with the murder. The three men, who were known to police, have denied any wrongdoing.

Police started laying out their evidence against the trio to a Malta court which will decide whether to order a trial.

One of the chief investigators, Keith Arnaud, described how a team from the U.S. Federal Bureau of Investigation, brought in by the Malta government to help solve the crime, had focused on a phone number which received a text message at the time of the explosion in Bidnija.

Police say the number was not attached to a mobile phone but to a circuit board used in remote-control devices. The appliance was switched on at 2 a.m. local time in Bidnija on the day of the explosion and went off the grid at the time of the blast.

Part of the circuit board was later found in the wreckage of the car.

Cell phone data suggested the device was triggered by a call from off the coast. CCTV video showed a boat owned by the Degiorgio brothers putting to sea at around 8 a.m. on the day of the killing. It was later seen still at sea at 2.50 p.m. and was idling at the time of the explosion before returning to harbor.

The three suspects had never been the target of any of Caruana Galizia’s often fierce blogs, leading to questions over their possible motive. The blogger’s family say the people who ordered the killing remain at large.

Arnaud revealed that police had already been tapping the phone of George Degiorgio at the time of the murder and had heard him asking two separate people to top up the credit of a mobile phone number on the morning of the blast.

He did not go into further details, but local media have reported that the same number might have triggered the bomb.

Arnaud was the first person called to testify in the pre-trial hearing, which has been delayed twice over the past week after the initial two magistrates assigned to the case recused themselves.

A third magistrate, Claire Stafrace Zammit, rejected calls by the defense for her also to abstain from the case because Caruana Galizia had once praised her husband. She dismissed the request as frivolous.

The hearing will resume on Wednesday.