On early Saturday morning off the coast of Japan, the Philippines-flagged cargo container carrier ACX Crystal struck the USS Fitzgerald (DDG-62) on its starboard (right) side, crushing the part of the Fitzgerald's superstructure where its commanding officer's quarters were and rupturing the ship's hull below the waterline. Seven sailors died in a flooding berthing compartment, and the captain (who was in his quarters) and two other crewmen were injured.

As the incident was unfolding, the world was given an almost immediate look at part of the story behind the collision thanks to data from the Automated Identification System (AIS) aboard the Crystal. AIS, a tracking system that has become the "Internet of Ships," was intended to help prevent such collisions, but it has also become a tool for nearly anyone to identify and track ships traveling around the world through websites and mobile applications. And the half of the story that Crystal's track told quickly raised questions about what exactly was going on with the freighter just before the collision—and whether the incident was something more than just a random accident.

AIS was developed in the late 1990s as a radio-based transponder system, initially intended to be used as part of a collision avoidance system for ships operating out of range of land-based shipping controllers. AIS has been extended further by the addition of satellite monitoring of AIS traffic and the integration of AIS data into navigational beacons and local vessel traffic services (VTS)—think air traffic control for ships. Mandated for all ships over 300 gross tons starting in 2002, nearly all commercial sea-going vessels are now required by one authority or another to be equipped with AIS for tasks such as fishing fleet monitoring, search and rescue, and maritime security. It also can be used for accident investigation along with the Voyage Data Recorder (VDR) "black box" mandated by the UN's International Maritime Organization. VDR is limited to 12 hours of data storage.

That data is also sent to the Internet through land-based sensors, and it is available through websites and mobile applications through companies such as ExactEarth, Vessel Finder, and Marine Traffic. And in the wake of the news of the collision on Saturday, posts from those sources rapidly fueled speculation about what caused the collision.

Based on @MarineTraffic it looks like something big happened with the ACX Crystal right around the time of the incident. #USSFitzgerald pic.twitter.com/EujOavynxX — 🌹 gregorous (@imgregorous) June 17, 2017

This is the unusual route ACX Crystal was sailing at around the time of the collision with USS Fitzgerald. Obtained using @MarineTraffic pic.twitter.com/vBMvuE6S76 — Greg Atkinson (@GregAtkinson_jp) June 17, 2017

Much of the speculation early after the collision focused on the erratic course changes by the Crystal just before the reported time of the collision. And there is still some dispute about the timing.

The problem, however, is that the AIS data tells only half, or less, of the story. That's because the Fitzgerald had its AIS system turned off, and it was not broadcasting its position and course. That means there's no current public record of the Fitzgerald's actions prior to the collision, and the crew of the Crystal would not have received a collision alarm from the system on their bridge.

Navy ships are equipped with AIS systems, as any vessel-tracking site will show. However, commanders of ships in the US Navy (and other navies) frequently turn off AIS to preserve mission security, as the Navy pointed out when Google warned the Navy of the security risks of AIS in 2012. And in many cases, that means that AIS is turned off whenever Navy ships are outside of the control area of local vessel traffic systems—whenever they're in open ocean. And merchant ships have been recorded turning off AIS before entering sensitive areas to avoid giving off location data, either for safety or for nefarious purposes.

Part of the initial suspicion about the data was caused by the reported time of the accident: the collision was widely reported to have occurred at 2:30am local time. But the Japanese Coast Guard reported that the report came long after the actual collision; the Fitzgerald's primary radio room was apparently taken out (though some communications were intact, and the ship was able to respond to communications later), and the Crystal did not report the collision to the Japanese Coast Guard until a full hour after the actual collision at 1:30am. In fact, after initially steering away from the collision, the Crystal did not turn around to render aid to the Fitzgerald for a full half hour, based on track data. (That's not necessarily out of the norm for such a large ship, as it could take miles to slow and turn around.) The US Navy reported the incident as having happened at about 2:20, however.

The Jiji Press news agency reported that the captain of the Crystal said his ship was "sailing in the same direction as the US destroyer and then collided." The collision occurred after the Crystal made an adjustment of course to port (left), steering toward the Fitzgerald's track. Under normal conditions, the crew of the ship showing its starboard side in a meeting situation is supposed to give the right-of-way. The other ship is required by the international "rules of the road" for shipping to maintain a predictable speed and course.

The bridge watch on the Fitzgerald apparently missed the course change by the Crystal or misjudged the speed of the ship and thought that the destroyer would pass ahead of the freighter. That problem could have been avoided if the Fitzgerald's AIS was turned on.

It's important to understand that the area in which the collision took place—off the coast of Japan, south of Yokosuka Naval Base and Sagami Bay, a gateway to maritime traffic headed for Tokyo—is immensely busy with shipping traffic.

As someone who has been on a bridge watch under those conditions prior to AIS, I can say that it's all too easy to become overwhelmed with tracking all of the boats around you, even with the assistance of radar. And even in open ocean without heavy traffic, things can go wrong quickly. Bridge watches have been described as "hours of boredom punctuated by moments of terror." In 1987, I was Combat Information Center watch officer aboard the USS Iowa (BB-61) when another ship steered toward us during an exercise and came dangerously close to a collision despite repeated warnings on ship-to-ship radio. (The captain and bridge watch officers on the other ship were fired the next morning by their squadron commander, who took over command temporarily.)