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The sons of slain investigative journalist Daphne Caruana Galizia are calling for the prime minister's resignation, saying in a Facebook post that he should show political responsibility for "failing to uphold our fundamental freedoms."

The sons wrote Thursday that they refused to endorse Prime Minister Joseph Muscat's call for a reward to lead to their mother's assassins, saying "we are not interested in justice without change."

Caruana Galizia, a harsh critic of Muscat's who reported extensively on corruption on the island nation, was killed by a car bomb on Monday.

Her sons wrote that identifying their mother's assassins was not enough — corruption on the Mediterranean island nation also needed to be rooted out, they added.

Muscat has denounced her assassination, and has proposed a reward to find her killers.