STOCKHOLM - British-flagged oil tanker Stena Impero is still being held in Iran, despite Tehran lifting a detention order on the vessel, its owner said on Wednesday.Sweden's Stena Bulk said it was not in negotiations with Iran and not aware of any formal charges against the crew or the company.Iran's foreign ministry said earlier that a lifting of the detention order had been finalized, but that an investigation into the vessel was ongoing."We haven't been accused of anything. Not through any formal letter or anything else to the company. We are still in the dark over why we are anchored in Bandar Abbas," Stena Bulk CEO Erik Hanell told Reuters by telephone.He said the ship was fueled and ready to sail for a port in the United Arab Emirates as soon as it was cleared to leave.Iranian forces seized the Stena Impero on July 19 for alleged marine violations two weeks after British marines detained an Iranian tanker off Gibraltar. The Iranian vessel was released in August.The actions followed attacks on other merchant vessels in Gulf oil shipping routes which Washington blamed on Tehran. Iran has denied responsibility.Hanell said tensions in the region had seen the company tighten security around its ships."Our British-flagged ships are now escorted by British frigates. We have a higher level of risk management now," he said.The head of Iran's Ports and Maritime Organization, Mohammad Rastad, said on Monday the Stena Impero was free to depart from Bandar Abbas port after legal hurdles had been cleared.Hanell said there were still Iranian guards on the ship and he had no idea why the ship had not been allowed to leave yet."We had a dialog with them up until the end of last week. We thought everything was ready (for the ship to be released)," Hanell said.Iran freed seven of the 23 crew members earlier this month and Stena Bulk has tried to get the rest out as well, in part by offering a crew exchange."We thought it would have been reasonable, but we weren't allowed to do it," Hanell said.