US officials said today that British sailors had boarded an Indian-flagged commercial ship suspected of carrying smuggled cars when they were seized last week by Iranian naval forces in the disputed Shatt al-Arab waterway.

It was the first detailed account of the operation being conducted by the 15 British sailors and marines when they were seized on Friday by Iran, setting off a standoff between Britain and Iran.

“It was an Indian-flagged vessel. It was suspected of being involved in automobile smuggling (into Iraq),” said Cmdr. Kevin Aandahl, a spokesman for the US Fifth Fleet, in a telephone interview from his headquarters in Manama, Bahrain. “It matched a profile of something we had told to be on the lookout for. It turned out to be not what we suspected and it was let go,” he said.

Iran contends the 15 were in Iranian territorial waters at the time, but Britain has said they were in Iraqi waters and should not have been seized by Iran. Aandahl said the captain of the Indian ship had provided a statement that his vessel was in Iraqi waters at the time it was stopped by the British. He said US officials knew the GPS coordinates of the ship, but would not release them publicly, and said the matter was being handled by the British government.