A fatal collision between a U.S destroyer and a Philippine container off Japan's Izu Peninsula in 2017 was caused by U.S. crewmen failing to follow lookout protocols, Japan's transport safety authorities concluded Thursday.

A report by the Japan Transport Safety Board said the watch team on U.S. destroyer Fitzgerald was unable to understand the intentions or actions of the ACX Crystal because it was concentrating on another vessel in the vicinity. Additionally, radar information about the Philippine ship was incomplete.

In compiling the report, Japanese authorities were not allowed to directly interview crew of the U.S. Navy ship. Seven American sailors were killed and three others on the destroyer, including the captain, were injured.

At around 1:30 a.m. on June 17, 2017, the 29,060-ton container ship, proceeding northeast, and the 8,261-ton missile destroyer, navigating to the south, collided in the Pacific Ocean off the tip of Izu Peninsula, Shizuoka Prefecture.

The Fitzgerald was flooded due to holes and other damage sustained to the starboard midship front shell, while the Crystal's port bow bulwark was damaged, the report said.

(U.S. destroyer Fitzgerald)

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At the time of the accident, three watch officers on the Fitzgerald were monitoring radar and conducting lookout only by eyesight.

While they confirmed the presence of the Crystal at around 1:20 a.m., the Fitzgerald maintained course and speed because it was paying attention to another approaching vessel that was parallel to, and north of, the Crystal.

Due to insufficient adjustment of radar, it became difficult for the Fitzgerald crew to confirm the presence of vessels within 3 to 5 kilometers on its monitor. The collision happened while the Fitzgerald was turning to starboard, according to the report.

The Crystal used daylight signaling lamps while keeping its course and speed, but the Fitzgerald did not recognize the signal or try to avoid the container.

Japan's safety board was only able to interview crew members of the container ship, and compiled the document based on the November 2017 U.S. Navy report on the collision, and information from the U.S. Coast Guard, which was entrusted to probe the accident by the U.S. Navy and the National Transportation Safety Board.

"We were able to investigate the accident thanks to cooperation among three countries, including the Philippines," a Japan Transport Safety Board official said. "(The probe) went relatively smooth."

The U.S. Navy report said the Fitzgerald's watch teams "disregarded established norms of basic contact management and, more importantly, leadership failed to adhere to well-established protocols in place to prevent collisions," concluding the accident was avoidable.

The Navy announced systemic reforms to restore fundamental naval skills and operational safety after the collision.