The husband of murdered investigative journalist Daphne Caruana Galizia has said he believes the architect of her murder is being protected by members of Malta’s government.

Six months after the brutal killing of his wife with a car bomb, Peter Caruana Galizia said political interests were interfering in the investigation into her death.

He said the three men currently awaiting trial over the killing were “simply contractors” and he feared the mastermind behind the attack would never be brought to justice.

“It is clear to us that the three men arraigned so far are simply contractors commissioned by a third party,” he told The Guardian.

“My sons and I are not convinced that our government really wants to establish who sent them, for fear such persons are in fact very close to our government.” For this reason, the truth may never emerge, he added.

The three suspects, Vincent Muscat and brothers Geroge and Afred Degiorgio, were charged in December last year. They all entered not guilty pleas. Police believe the person who ordered the bombing is still at large.

Malta’s prime minister Joseph Muscat has promised to leave no stone unturned in finding who ordered the killing and the country’s justice minister has offered a €1m (£870,000) reward in exchange for information leading to their capture.

The government has said the murder is being investigated “vigorously” and ”police will have whatever resources they need to pursue and prosecute those responsible”.