The police were closing in on the suspicious car seen at a vantage point overlooking the road where Daphne Caruana Galizia was killed, but it was still unclear whether this would prove useful in the investigation, sources said.

They said the efforts to identify the car were at an “advanced” stage.

Times of Malta reported last week that investigators did not exclude the possibility that the car was used by the person who detonated the powerful explosive device.

It was seen parked in the middle of a country lane some distance from nearby buildings and raised suspicions among motorists in the area.

A description of the car, which was rented, was given to the police. The first three letters of its registration plate were also noted. Tracing it was, however, proving difficult, because it was a "particularly popular model", the sources said.

A detail that could prove crucial was a sticker on the back of the car, which had helped to narrow down the search, they added.

"When the search for the car started, we were looking at thousands of possible suspect vehicles but now we have narrowed it down to a few dozen," the sources said.

They pointed out that even if the car was found, its relevance to the investigation would still need to be established. DNA samples could be taken from it, and CCTV footage and documents relating to the time it was rented could be examined.

However, it was still far too early to tell, they said.

The area where the car was spotted was cordoned off by the police and combed extensively by a team of forensics experts.

Cigarette butts and other items that could serve as evidence were collected from the area and sent for DNA testing.

The funeral of Ms Caruana Galizia will be held tomorrow and will be marked with a national day of mournig.