"I think that this much is clear, there was a navigational error of some kind," he said at a press conference on Thursday. "All the contributing factors to that we don't know yet, and we're still talking to those folks, and we'll find out more ... but they were clearly out of the position that they intended to be in."
The two U.S. Navy boats seized by Iran with 10 sailors
on board were within three miles of Iran's Farsi Island when the Iranians took them, according to a U.S. defense official directly familiar with the latest information about the incident.
The boats had drifted off course, but one was also suffering engine trouble, making it impossible for the Americans to rapidly back off and return to international waters when they were approached by armed Iranian naval boats, the official said.
The U.S. Navy has initial statements from the sailors but is now conducting a full debrief, so additional information may come to light, the official said.
While the sailors appear to be fine physically they are "tired and upset" about what happened, according to the official.
One of the boats began to have its engine "run roughly" during the transit from Kuwait to Bahrain, according to the details of what the crew has said. It's not yet clear at what point they drifted off course, or if they even understood they were off course. It's not believed the engine trouble was the cause of going off course.
But when they got so close to Farsi Island, and were approached by the Iranians, it is believed the U.S. boats could not rapidly get out of way because of that engine trouble.
Iranian territorial waters extend for three miles around Farsi Island, according to the official. The boats were supposed to stay out of that area. Farsi Island is a naval base for Iran's naval component of the Revolutionary Guard Corps and is considered extremely sensitive.
Commanders in the U.S. Navy's Fifth Fleet in the region became alarmed when the sailors missed a planned check-in call on their radios. At that point, commanders checked the GPS system tied to the boats and realized they were off course and inside Iranian waters.
A search-and-rescue effort was launched -- including sending a U.S. Navy vessel inside Iranians waters due to concern the sailors could have been overboard and in the water. The U.S. Navy informed Iranian military vessels in the area they were coming in for a search-and-rescue, the official said. There were "robust bridge-to-bridge communications" during that time, but there was no trouble, the official said.
But it then became known the 10 sailors had been taken ashore and diplomatic efforts to get them out quickly kicked in. The Pentagon is trying to confirm reports that one of the U.S. sailors spoke on bridge-to-bridge radios to the U.S. Navy during the incident.
Both boats were returned to the U.S., and the Navy does not believe the Iranians removed any gear or weapons, the official said.
The U.S. Navy unit was scheduled to return to its home port in San Diego in the next few weeks.
The airing of footage of the incident, which shows the sailors on their knees with their hands up while surrounded by armed Iranian Revolutionary Guards, has generated controversy, with Carter telling reporters Thursday, "Obviously I don't like to see our people being detained by a foreign military."
Carter, speaking at U.S. Central Command headquarters, said he would defer judgment until the sailors could be debriefed fully, saying, "I want to give them the chance to tell us what they saw."
The White House told reporters Thursday that President Barack Obama had seen the images of the detained sailors.
With the administration on the defensive over the video, State Department spokesman John Kirby told CNN's Jake Tapper on "The Lead" that the images were "hard to look at" but urged critics to concentrate on the fact that the sailors were returned quickly.
"What we are most happy about here in the State Department is that we were able to get them home in less than 24 hours, (with) 10 fingers, 10 toes, nobody hurt. They are all safe and we got our boats back, and I think that is the most important thing."