WASHINGTON » Ten U.S. sailors detained by Iran last week were taken at gunpoint after straying into Iranian waters, the Defense Department said in the first formal account of the confrontation in the Persian Gulf.

“Armed Iranian military personnel” boarded the U.S. boats while others “conducted armed over-watch of the boats with mounted machine guns,” U.S. Central Command said in the preliminary report issued Monday.

It describes an incident on Jan. 12 that began as a tense confrontation but ended quietly, with no indications the sailors were physically harmed.

Sailors on two Riverine Command Boats traveling from Kuwait to Bahrain “deviated from their planned course” down the middle of the Persian Gulf, according to the report, which said the reason remains under investigation. The boats stopped for troubleshooting after indications that the diesel engine on one of the vessels had a mechanical problem. “The stop occurred in Iranian territorial waters, although it’s not clear the crew was aware of their exact location.”

Iranian vessels approached, and initial reports indicate “there was a verbal exchange between the sailors and the Iranians but no exchange of gunfire.”

The incident, which happened hours before President Barack Obama delivered his annual State of the Union address, threatened to deepen rifts between the U.S. and Iran even as the two nations prepared to implement a landmark agreement to curtail the Islamic Republic’s nuclear program.

Republican presidential candidates and members of Congress quickly criticized Obama for the capture, calling it another indication of what they consider a feckless strategy toward Iran and Mideast militants. The administration said the rapid release of the sailors, who were held overnight, showed the benefits of improved diplomatic relations with Iran.

“At this point there are no indications that the sailors were physically harmed during their detainment,” the Pentagon said. “The Navy command investigation will focus on the sailors’ treatment while in Iranian custody, including any interrogation by Iranian personnel.”

Videos released by Iran showed the sailors forced to get on their knees with their hands behind their heads when they were captured and one of the Americans apologizing for the incident. Later in the video, the Americans are shown eating an Iranian meal.

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