Gibraltar police said on Friday that all four crew members of the Iranian Grace 1 tanker have been released on bail without charge after it was detained last week on suspicion of breaking European sanctions by taking oil to Syria.
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.
In a statement, the police said all four crew members were granted bail with conditions. It did not elaborate, but said the investigation was ongoing and that the Grace 1 continues to be detained.
Also Friday, Britain said it would deploy the destroyer HMS Duncan warship to the Gulf to replace HMS Montrose, maintaining a continuous presence there.
"As part of our long standing presence in the Gulf, HMS Duncan is deploying to the region to ensure we maintain a continuous maritime security presence while HMS Montrose comes off task for pre-planned maintenance and crew change over," the government said.
"This will ensure that the U.K. alongside international partners can continue to support freedom of navigation for vessels transiting through this vital shipping.
Tension between Iran and the West has increased a week after Britain seized the tanker and London said the British Heritage, operated by oil company BP, had been approached in the strait between Iran and the Arabian peninsula.