The 10 American sailors were on an unfamiliar assignment and running behind schedule.

Setting off from Kuwait en route to Bahrain, the U.S. sailors had never navigated across the Persian Gulf in small patrol boats, and they were unaccustomed to traveling such a long distance in vessels designed for shorter missions in coastal waters or rivers.

Their routine mission on Jan. 12 turned into a nightmare when they strayed into Iranian waters near Farsi Island in the Persian Gulf. Members of Iran’s Islamic Revolutionary Guard Corps (IRGC) surrounded the two U.S. boats and made the crew members kneel in surrender with their hands on their head. Iran released a video of the humiliating scene and held the sailors for 16 hours, before a flurry of phone calls between top diplomats in Washington and Tehran secured their release.

The incident, which remains under military investigation, has raised troubling questions not only about the crew’s performance but also about the U.S. Navy’s operations and readiness in one of the world’s most strategic and volatile waterways — where American ships have sailed for decades.

Mechanical problems, communication breakdowns, and a lack of navigation training or preparation all played a role in the blunder, Foreign Policy has learned, based on interviews with officials and others familiar with the case.

Coming just days before a landmark nuclear agreement with Iran entered into force, the capture of the sailors carried the potential to escalate the situation into a full-blown international crisis. And the incident turned into a political firestorm in Washington. Critics of President Barack Obama’s approach to Iran have seized on the administration’s handling of the incident and its aftermath, citing it as further proof that Washington is pulling its punches with Tehran and withholding details from lawmakers.

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Officials told FP that the results of an initial U.S. Navy investigation into the case were turned over this week to the commander of the 5th Fleet, Vice Adm. Kevin Donegan, who oversees naval forces in the Middle East. The commander has 30 days to review the findings, decide if more investigatory work is required, and recommend if the crew members or their superior officers should be prosecuted or reprimanded. Other senior officers will then review those recommendations, culminating with a final decision by the Navy’s chief or deputy chief.

Several people — including Obama administration officials, congressional staffers, and others familiar with the details of the incident — discussed the case with FP on condition of anonymity due to the ongoing investigation.

In the meantime, Senate Armed Services Chairman John McCain (R-Ariz.) has accused the administration of dragging its feet and demanded that it provide a full briefing to lawmakers. Otherwise, he says, he will subpoena the sailors and potentially drag them into embarrassing public hearings.

The sailors in Kuwait who were captured by the Iranians were trained to operate riverine command boats, or RCBs — small, speedy craft about 50 feet long that are used to transport special operations forces, patrol coastal waters, or escort larger ships. The sailors, under the command of 27-year-old Lt. David Nartker, were ordered to Bahrain to take part in an exercise and had less than 24 hours to prepare. Only one of three boats at their disposal was in working order. The crew members had to cannibalize one of the broken vessels to get an engine part so they could have a second boat to sail.

The trip required the crews to travel about 240 nautical miles, more than twice the usual distance they were accustomed to. That meant the two riverine boats would have to refuel at sea roughly midway through the journey, said one U.S. official who confirmed details of the trip.

Due to problems getting their communications gear to work, the sailors set off from Kuwait three hours late. The late start put pressure on the sailors as they tried calculate how long before they would need to refuel in daylight and the optimum speed for their boats, according to a second person familiar with the sailors’ accounts.

The boat crews asked that a refueling tanker meet them before nightfall, as they were not trained for refueling in the dark. Arranging a rendezvous proved difficult because the sailors had no direct communication with the tanker and had to relay messages through a U.S. operations center in an undisclosed location in the region. As the boats sought to meet up with the refueling tanker, they mistakenly ventured into Iranian territorial waters, just west of tiny Farsi Island.

The U.S. sailors were using a GPS device to navigate, but Farsi Island is so small that it did not appear on their screen when it was zoomed out to a wider view. As they drifted within sight of land, the Americans did not even know that it was Farsi Island, said the person familiar with the sailors’ account.

Throughout the cruise, the positions and direction of the two boats were automatically relayed to the operations center every 30 minutes via an electronic tracking device, the U.S. official said. But for reasons that remain unclear, commanding officers or others at the operations center did not inform the boat crews that they were headed in the wrong direction.

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Traveling at a swift pace, the two U.S. vessels might have passed through the area without encountering any Iranian patrol craft. But one of the American boats — the one that needed repair back in Kuwait — broke down. As the sailors worked to revive the boat’s engine near Farsi Island, two IRGC patrol boats showed up, weapons pointed at the Americans. One American sailor waved a wrench in the air, to signal they had engine trouble and had no hostile intent. But the Iranians showed no interest in helping out: Soon another Iranian vessel arrived at the scene, followed by a fourth ship that was larger and more heavily armed.

As the Iranians encircled the boats, the U.S. sailors managed to repair the faulty engine. Now the Americans had a choice. With 50-caliber machine guns and GAU-19 miniguns on their boats, they outarmed the Iranians. And their RCBs were bigger than the Iranian patrol craft. But escaping would mean opening fire on the Iranian forces or ramming their vessels — actions that could lead to a wider conflagration. The young officer in charge, Nartker, opted to cooperate.

In the tense encounter, there was one comical moment. As the Iranians searched through gear and equipment, they held up an iPhone 6 charger with an accusatory air. It took some time for the U.S. sailors to convince the Iranians that it was merely a charger, featuring a new design for the latest iPhone, and not a hi-tech weapon.

After being ordered to kneel with their hands on their heads, the Americans were taken to nearby Farsi Island and questioned one by one. They were served meals. At one point, the food was removed and served again so the scene could be recorded for the camera. Although the sailors were blindfolded and used as propaganda for Iranian state television, the Americans said they were not physically abused or otherwise mistreated.

Meanwhile, back at the U.S. operations center, commanders saw that the tracking device for the riverine boats had put them in Iranian waters near Farsi Island. The U.S. commanders ordered a cruiser, the USS Anzio, a U.S. Coast Guard cutter, and a British naval ship to head over to the boats and lend them assistance. But the timing of the response — and whether commanders were aware that the American boats had encountered Iranian patrol craft — remains unclear. By the time the rescue party arrived, the U.S. sailors were already in the hands of the Iranians.

After Secretary of State John Kerry and his Iranian counterpart, Mohammad Javad Zarif, worked out the release of the Americans in a series of phone calls, the sailors were told that they were about to be freed. The Iranians filmed one crew member crying in relief at the news.

Read more at Foreign Policy