"This incident was the result of failed leadership at multiple levels from the tactical to the operational," investigators wrote in the detailed, partially redacted, report.
The report found the crews were poorly prepared, their boats not properly maintained, communication almost entirely lacking, and their conduct after being captured by the Iranians wasn't up to military standards.
In a stunning finding, the report said the sailors veered off course almost immediately after heading out to sea and had no idea where they were when a mechanical failure struck one of the boats.
"The boat crews could visually see Farsi Island, but were not concerned as they were unaware that it was Iranian or that they were in Iranian waters," the report said.
The report details a lax culture for U.S. Navy sailors who routinely patrol the Persian Gulf which ultimately led to a highly embarrassing incident for the U.S. military just as crippling economic sanctions were set to be lifted as part of the Iranian nuclear deal.
"The culture ... (was) characterized by informality. They conducted no patrol briefings, and missions were supported by no formal mission analysis, standard planning factors, risk assessment, or overwatch," investigators wrote.
At a news conference to release the report, Chief of Naval Operations, Adm. John Richardson, said, "This will be a case study going forward. There are lessons that apply across our entire Navy."
One of the only bright spots noted in the report was a sailor who "showed presence of mind and fighting spirit when she attempted to activate" at tracking beacon at some point during the incident.
And after the U.S. crew members were captured, more mistakes were made. The report found that during the 24 hours they were held some crew provided more information to their Iranian captors than they should have, and that they ate food while being filmed -- something they should not have done because it can be and was used as propaganda. One crew member disobeyed a direct order, the report said.
Asked by their captors how it was possible a boat like theirs could have traveled such a distance, one sailor replied, "Yeah, I wish you could tell my people that because we told them these boats don't do that" -- a statement investigators said was inappropriate.
The report concluded, however, that the Americans didn't violate international law, while the Iranians did.
"The investigation concluded that Iran violated international law by impeding the boats' innocent passage transit, and they violated our sovereign immunity by boarding, searching, and seizing the boats, and by photographing and video recording the crew," Richardson said at the news conference.
In their report, investigators called the mission a "complex transit" of 259 nautical miles from Kuwait to Bahrain that required more than the 24-hours advance notice the crews were given.
"Essentially, there was no time given for the team to think through the task before executing. The collective team felt a sense of urgency for a mission that had previously been rescheduled and had no required accomplishment date," the report said.
The Iranian Revolutionary Guard captured the sailors January 12
after an engine died on one of their two boats. As the sailors waited for repairs, the Revolutionary Guard approached in several boats and took them captive with guns drawn.
"The engine casualty in Iran's territorial seas is the culmination of failures in multiple areas, including maintenance, personnel qualification, sustainment training and crew rest," they wrote.
After the sailors had breached both Iranian and Saudi Arabian territorial waters and been forced to stop, the sailors did not have a plan for communicating their location or progress with officers.
"There was no coherent plan to communicate with the craft of plot their progress in relation to the approved navigation plan," investigators found.
The capture was elevated to stunning prominence as Iranian television broadcast footage of the sailors being held at gunpoint on the same night as President Barack Obama's final State of the Union address. The sight of U.S. sailors with their hands behind their heads being held by Iranian military raised sharp criticism from Republicans who were already firing away at Obama for his handling of ISIS in the Middle East.
The report also found that the crew was never familiarized with the region, and didn't know about weather, geography or potentially hostile threats.
In addition, before going out to sea, there's supposed to be a written patrol briefing. But personnel couldn't recall seeing that, the report said, and the investigation couldn't find it and questioned if it had existed.
The U.S. craft were also undermanned, and couldn't be operated at the same time the weapons were being manned.
"The investigation found a lack of leadership, a disregard for risk management processes and proper mission planning standards," Navy Vice Adm. John Aquilino, deputy chief of operations, said at the news conference.
The report said that mission leaders showed "blatant disregard for the genuine concern of sailors," not listening to their concerns or empowering them.
Three officers have already been disciplined and five or six sailors on the Coastal Riverine squadron may also be punished.