U.S. Navy handout photo of a riverine patrol boat from Costal Riverine Squadron 2 escorts the guided-missile cruiser USS Bunker Hill while in the Arabian Gulf.

A navigational error caused two U.S. Navy vessels to stray into Iranian waters, Pentagon chief Ashton Carter said Thursday, as opponents of the Obama administration voiced increasing anger over the U.S. response to the incident.

Carter provided no detail but said “a navigational error of some kind” led to the two small patrol boats ending up in Iranian water on Tuesday. The military had initially blamed mechanical problems.

The vessels were apprehended by Iran’s Revolutionary Guards while en route from Kuwait to Bahrain. Ten sailors were held overnight before being released back to the United States.

“We will find out more what combination of factors led to that navigational error, but they were clearly out of the position that they intended to be in,” Carter said.

Critics of the administration of President Barack Obama are livid over the affair, saying it demonstrates weakness in the face of Iranian aggression.

Video showing the U.S. sailors kneeling and with their hands clasped behind their heads, and one of them apologizing, aired on U.S. television Wednesday, further angering lawmakers.

Carter, speaking at a news conference in Florida, deflected questions on whether Iran had breached the Geneva Convention by showing images of the captured sailors.

“Obviously I don’t like to see our people detained by a foreign military,” Carter said.

“What we don’t know is the full context. What you are looking through is the lens of the Iranian media. We need to give these guys the opportunity to tell us what was really going on and what the overall context is before we can really know.”

Video also showed Iranian troops rummaging through an array of weapons found aboard the US boats.

General Lloyd Austin, who oversees the US military’s Central Command, said the Iranians had “largely” returned the US equipment.

“For the most part, the gear we deployed with was largely there when we got the boats back,” he said.

“Whether or not there was a singular piece of equipment missing, we’ll determine that once we have completed the inventories,” he added.

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