US sailors blundered their way into hands of Iranians, says Navy report

US sailors who were captured in January after straying into Iranian waters committed basic navigational blunders, were quick to surrender and some buckled under questioning, according to a damning navy report released on Thursday.

The report of the 12 January incident in the Persian Gulf highlights a string of leadership and procedural failings in Iran's humiliating seizure of the 10 sailors, with investigators saying the crews of the two captured boats were derelict in their duties.

The sailors at times appeared hopelessly lost at sea, with the engineer on one boat even resorting to an app on his phone to try to determine the name of an unexpected land mass - which turned out to be Iran's Farsi Island - only to find his phone displaying a "long Arabic name" and no other information.

In all, nine navy personnel - three of whom were actually on the boats that were meant to head from Kuwait to Bahrain - have been disciplined or will face disciplinary action.

Iranian media broadcast humiliating images of the US sailors during their detention, showing them kneeling on their boats at gunpoint with their hands on their heads.

At one point during their arrest, a gunner from one boat thought about trying to escape, but "did not think of himself as a prisoner of war because the conditions were too nice," the report said.

Though the sailors were held for less than 24 hours, the incident was a major embarrassment for the US Navy and for President Barack Obama.

The US avoided escalating the situation, maintaining a conciliatory tone with Tehran just days ahead of the implementation of a historic international deal over Iran's nuclear programme.

"I didn't want to start a war that would get people killed," the crew's captain told investigators in explaining why he had quickly surrendered.

"I made the gamble that they're not going to [take us to] Tehran and parade us like prisoners of war, because they want this nuke deal to go through."

Name, rank, serial number

Some of the crew members told investigators they were subjected to "aggressive behaviour" by the Iranians, such as an interrogator "slapping his hand down on the table, spinning a crew member's chair around or threatening to take a crew member to Iran for lying."

Sailors told Iranians their names, ranks, serial numbers and dates of birth. Some "played stupid" and evaded answering questions, but others revealed sensitive information such as passwords to their phones and laptops, the report said.

The Navy has slammed Iran for arresting its sailors, for hampering their "innocent passage" of Farsi Island and for using them as propaganda tools when it filmed them surrendering and while in captivity.

But the investigation "found a lack of leadership, a disregard for risk management processes and proper mission-planning standards," said Chris Aquilino, a US vice admiral.

"If the guidance provided... had been followed, this event could have been prevented."

The ill-fated mission was destined for trouble from the beginning, the report said.

Originally, the two boats were supposed to go from Kuwait on a 480km trip to Bahrain, the longest such sailing the crews had ever conducted.

But the mission got off to a late start, so the crews attempted to take a shortcut to make up time, investigators found.

This "deviation" caused them to unknowingly enter Saudi Arabian waters, and then stray into Iranian territorial seas off the coast of Farsi Island. The sailors saw "land masses," but did not realise where they were.

"Crew members speculated as to whether these were Saudi islands, rocks or oil platforms," the report said. "None of the crew believed that any of the masses were an Iranian island."

The crews had functioning navigation systems, but failed to zoom in to the right level of magnification, the report said.

Less than two miles from the island, one of the boats broke down and stopped in the water.

Gun ports open

As crews worked to fix it, two Iranian Revolutionary Guard Corps Navy vessels approached with their guns uncovered. Shortly after, the Americans fixed the mechanical problem and attempted to flee, only to be blocked.

The crews would have been operating within rules of engagement to offer armed resistance, but the captain decided to surrender in order to de-escalate tensions.

The 10 sailors were taken to Farsi Island, where they were interrogated and spent a fitful night.

Eventually, the crew's officer made a scripted apology, saying the incident was the Americans' fault and that the Iranians' behaviour had been "fantastic."

The Navy has already relieved three officers of their commands, one of whom was on one of the boats.

Six other sailors in the case face non-judicial punishment.