Photo taken on Jan. 25, 2013 shows a tourist telescope seen at the Rock of Gibraltar in Gibraltar. (Xinhua/Wang Lili)

LONDON, July 13 (Xinhua) -- All four crew members of Iranian oil tanker Grace 1 have been released on bail without charge, the Royal Gibraltar Police said on Friday.

"All four crewmembers of the Grace 1 arrested by the Royal Gibraltar Police have been bailed without charge," said the Royal Gibraltar Police, the principal civilian law enforcement agency in the British overseas territory of Gibraltar, in a press release published via Twitter.

"The investigation is still on-going and the Grace 1 continues detained," it added.

The police said earlier on Friday that they had arrested the two second mates from the oil tanker, a day after the vessel's captain and chief officer were arrested.

Combined photo taken on Aug. 26, 2008 shows British marines on Royal Navy frigate HMS Kent at the Harumi Pier in Tokyo, capital of Japan. (Xinhua/Ren Zhenglai)

According to media reports, British Royal Marines in the Strait of Gibraltar last Thursday seized Grace I which was heading to Syria "in breach of European Union (EU) sanctions."

In response, Iran's Foreign Ministry urged Britain to release its oil tanker "soon," saying "the British claim over the oil tanker is legally void," Iran's official IRNA news agency reported on Friday.