In this Oct. 26 2015, photo provided by the U.S. Navy, Riverine Command Boat (RCB) 802, assigned to Combined Task Group (CTG) 56.7, conducts patrol operations in the Persian Gulf. Weak leadership, poor judgment, a lack of "warfighting toughness" and a litany of errors led to the embarrassing capture and detention by Iran of 10 U.S. sailors in the Persian Gulf in January, according to a Navy investigation released Thursday, June 30, 2016. (Torrey W. Lee/U.S. Navy via AP)
In this Oct. 26 2015, photo provided by the U.S. Navy, Riverine Command Boat (RCB) 802, assigned to Combined Task Group (CTG) 56.7, conducts patrol operations in the Persian Gulf. Weak leadership, poor judgment, a lack of "warfighting toughness" and a litany of errors led to the embarrassing capture and detention by Iran of 10 U.S. sailors in the Persian Gulf in January, according to a Navy investigation released Thursday, June 30, 2016. (Torrey W. Lee/U.S. Navy via AP) Credit: MC2 Torrey W. Lee

Washington — The 10 U.S. sailors captured and humiliated by Iran after mistakenly steering their boats into Iranian waters in January were beset not just by poor judgment and faulty equipment. They also showed a remarkable lack of curiosity about potential dangers in one of the world’s more dangerous waterways, according to an in-depth Navy investigation.

In deviating from their planned Persian Gulf route from Kuwait to Bahrain — without asking approval or notifying superiors — they passed an island to their east and wondered whether it might be Saudi territory, rocks or oil platforms. The crews of both boats consulted their navigation systems, which depicted the mass as a small purple dot.

Despite being unsure of their surroundings, the sailors did not adjust their on-board navigation displays to enlarge the purple dot; if they had, they would have seen that it was labeled Farsi Island, a well-known base for the Iran Revolutionary Guard Corps Navy.

“No crewmembers on either (boat) utilized a paper navigational chart in order to plot their exact location or to identify the island they had seen, even though the charts were available” on their boats, known as Riverine Command Boats, the investigation report said. No crewmember even bothered to log the fact that they had seen the island.

“Crewmembers lacked navigational awareness, proper communication with higher authority, and appreciation of the threat environment throughout the transit,” the report said.

The trouble for Riverine Command Boats 802 and 805, each with five sailors aboard, began even before they left port in Kuwait Jan. 12 on a short-notice, 300-mile journey to Bahrain. They were delayed, unprepared, poorly supervised and ill-suited for the mission, the report said.

At least one sailor had been up all night with boat repairs. Their higher headquarters failed to arrange air or surface monitoring of the boats’ transit. Such monitoring “would likely have prevented” the sailors’ capture by the Iranians, according to the report.

The Iranians boarded the U.S. boats, confronted the sailors at gunpoint and took them to Farsi Island, where they were fed, interrogated and kept overnight before being released after Washington intervened. The incident caused uproar in the United States, coming on the day of President Obama’s final State of the Union address. Republicans criticized the administration’s response, which included thanking Iran for releasing the sailors.

The investigation concluded that while the boat crews erred in entering Iranian waters, the Iranians violated international law by impeding the boats’ “innocent passage,” and violated U.S. sovereign immunity by boarding and seizing the boats.

Officials said that as a result of the incident, the Navy is stepping up training in adherence to the code of conduct.