Police in Gibraltar arrested the captain and the chief officer of a seized Iranian tanker that was breaking EU sanctions by carrying oil to Syria.
The two men, both Indian nationals, were arrested on Thursday, a spokesman for the Royal Gibraltar Police said.
"This follows a protracted search of the vessel where documents and electronic devices have been seized and examined," police said.
Neither men has been charged but police are still interviewing them.
British Royal Marines boarded the Grace 1 off the coast of Gibraltar on July 4 and seized it.
The government in Gibraltar, a British territory, said it believed the vessel was breaking EU sanctions by shipping crude oil to the Baniyas Refinery in Syria, which is under the control of Bashar Al Assad’s government.
The refinery's owner is on the EU's sanctions list.
British Foreign Secretary Jeremy Hunt, who is also in the race to become the UK's next prime minister, said developments were “very concerning” and that he was keeping an eye on the situation.
"We are constantly keeping under review the kind of security we need to keep British shipping safe," Mr Hunt said.
The move sparked fury in Tehran where the British ambassador was summoned to explain the “illegal interception” of the supertanker. Iran denies the tanker was headed for Syria.
The arrests come a day after three Iranian vessels trried to stop and divert an oil tanker, owned by BP, from its passage through the Strait of Hormuz.
A British naval frigate warned off the Iranian Islamic Revolutionary Guard Corps boats by aiming weapons at the vessels.
The Revolutionary Guard have denied any involvement in the incident but warned the US and Britain that they would "strongly regret" capturing the Grace 1.