The 10 U.S. sailors detained for a tense 16 hours on an Iranian island base are being debriefed in Qatar as more details emerge from the high-stakes encounter.

The boat crews were arrested by Iranian forces after they skirted by Iran's Farsi Island in the Persian Arabian Gulf. The sailors seem to have mistakenly entered Iranian waters aboard their top-of-the-line riverine assault boats. Their navigation systems appeared to be working and the boats had fuel.

Officials are trying to ascertain how they wound up off track, leading to the run-in with Iran's paramilitary forces based on the island, according to a source familiar with the debriefings.

A refueling rendezvous gone awry is the likely explanation for why the boats got off track, according to a U.S. official who asked for anonymity to discuss information from the debriefings. who spoke on background because he was not authorized to speak publicly on the matter.

In order to make the trek from Bahrain to Kuwait, the riverine boats needed to take on more fuel mid- have to refuel at some point in thejourney. The rendezvous point for the refueling was about three miles west of the 12-mile limit that rings Iran's territorial sea around to the west of the 12 nautical mile limit around Farsi Island. To get to the rendezvous, however, the boats deviated from an approved course and ended up in Iranian waters. 

"Nobody understand why they did what they did," the official said. "The way they went off course required some notification or approval that they didn't seek."

One theory is that  the crews took a short-cut decided to take an ill-advised short-cut to the refueling rendezvous point and couldn't outrun Iran's pursuit vessels fast boats escape because one of the riverine boats was suffering engine trouble that prevented them from going full speed. The jet-propelled riverine command boats are rated at speeds upwards of 43 knots, or 50 mph involved can travel in excess of 40 knots when operating correctly

The ships' guns, radio and navigational equipment were all returned with the boats. The official would not comment on whether any crypto was lost in the seizure, citing government secrecy rules. policy not to discuss such matters.

Top officials say they're still piecing together what led up to the confrontation at sea and the details of the 16-hour detention.

"I think it would not be a good thing to speculate on what may have happened," Gen. Lloyd Austin said in a press briefing Thursday. "We'll know a lot more after we finish debriefing our sailors and we're in the process of doing that right now."

Austin declined to discuss any gear the Iranians may have taken from the boats.

U.S. military investigators "are going through inventories right now, but for the most part, the gear that we deployed with was largely there when we got the boats back," Austin said.

It remains unclear "whether or not there are singular pieces of equipment missing," Austin said.

Officials have not said how they're handling the U.S. sailor who appears to have apologized to the Iranian captors in a propaganda video.

"It was a mistake. That was our fault. We apologize for our mistake," one unnamed sailor said to his Iranian captors.

The propaganda video was released by Iran's semi-official Fars news agency Wednesday. Images from these videos — including their surrender with hands on the backs of their heads on the boats — splashed onto front pages and homepages around the world.

Iran freed the sailors Wednesday. They left with their weapons and radios when they departed Farsi Island aboard their riverine command boats, 49-foot-long assault craft that are the flagships of the brown water Navy, which Iranian officials searched for advanced technology and sensitive communications.

Navy Secretary Ray Mabus said the nine men and one woman captured are going through a debriefing. 10 riverines captured by Iran earlier this week and returned the following day.

"They appear to be in very good shape. We've been in touch with all their families," he said. "We'll know more as they're debriefed."

Some of the crew members were exhausted and anxious after their detention but none were harmed during their 16-hour detention, according to the source familiar with the debriefing.

Having the sailors returned unharmed from Iran is a tremendous relief and the first-step towards their recovery and return to duty, said the Navy's enlisted leader.

"Right now, the No. 1 thing that I'm most happy about is that our sailors are back," said Master Chief Petty Officer of the Navy (AW/NAC) Mike Stevens in a Thursday interview with Navy Times. "As far as lessons learned, we're going to have to sort through that. As you can imagine, we're doing a thorough review and as that review is completed, we'll determine the next course of action that's needed."

The foremost priority, he continued, "is making sure that we're caring for the sailors and then we'll sort through everything else as time goes on."

"They'll go through a process of debriefing and making sure that their mental and physical and emotional needs are taken care of," he added. "Every sailor's going to be a little bit different."

Navy Times staff writer Sam Fellman contributed to this report.