The 10 US sailors who were held by Iran made a navigational mistake that led them into Iranian waters but did not communicate that to navy commanders before being intercepted, the US military says.
US Defence Secretary Ash Carter said the information came from debriefings of the sailors, who were flown on Wednesday (local time) to a US military facility in Qatar after Iran released them along with their two riverine boats.
"The information that they have given us, and through their commanders, is that they did stray accidentally into Iranian waters due to a navigation error," Carter said in an interview on Thursday with Fusion television's Jorge Ramos, which will also air on Spanish-language Univision.
Carter's comments were the most detailed so far from American officials on the incident, which rattled nerves just before the expected implementation of a landmark nuclear deal between Iran and world powers.
Diplomats in Washington and Tehran, through a series of anxious phone calls, sought to resolve the incident quickly, ensuring it did not torpedo the nuclear accord.
In the end, Iran released the US sailors - nine men and one woman - on Wednesday.
The speedy resolution of the incident contrasted with previous cases in which British servicemen were held considerably longer, in one case nearly two weeks.
Carter said the sailors apparently did not radio in to tell their commanders they were off-course before encountering the Iranians.
"They did not report this navigational error at the time. It may be that they were trying to sort it out at the time they encountered the Iranian boats and discovered they were inside of the territorial waters of Iran," Carter said in the interview.
He denied the sailors were on a covert mission, saying; instead, "they were simply transiting from one place to another".
One US official, speaking on condition of anonymity, said it was clear the boats did not run out of fuel.
A senior official said the navy was beginning the process of reintegrating the sailors after medical, psychological and mental health examinations.
Navy officials were also staying in close touch with the families of the sailors to keep them apprised of the situation, the official said.