In a statement on Friday, the police said all four crew members were granted bail with conditions, according to Reuters.
It did not elaborate, but said the investigation was ongoing and that the Grace 1 continues to be captured.
Gibraltar police said earlier on Friday that they had arrested the two second mates from the tanker, a day after the vessel’s captain and chief officer were arrested.
Last week, the British Royal Marines seized the Iranian oil tanker off the coast of Gibraltar for trying to take oil to Syria allegedly in violation of EU sanctions.
Tehran maintains that the supertanker was not bound for Syria and its seizure has taken place at the behest of the US.
Chief of Staff of the Iranian Armed Forces Major General Mohammad Hossein Baqeri has warned that the UK hostile measure will not go unanswered, saying, “If necessary, the response to such a shameful move will be put on the agenda, at an appropriate time and place.”