In the wee hours last Monday, American guided-missile destroyer USS John S. McCain sailed through the Singapore Strait, one of the world's busiest seas where vessels are sometimes separated by under a nautical mile, or about 1.8km.
At 5.24am, the warship, which comes under the United States 7th Fleet's command and was en route to a routine call in Singapore, collided with Alnic MC, an oil tanker carrying some 12,000 tonnes of fuel oil.
Five sailors were injured and another 10 went missing. The incident sparked a Singapore-led five-nation search which at one point covered an area of 5,524 sq km.Two bodies have since been found, with the others believed to be trapped in the warship which has since been moved to the Changi Naval Base.
Even as a probe is underway, questions have arisen over a statement by the Maritime and Port Authority of Singapore (MPA) to The Straits Times last Thursday, in which the MPA said it had detected only the tanker, not the US warship, before the collision.
This has cast a spotlight on the MPA's Vessel Traffic Information System (VTIS), and raised other questions on how commercial and military vessels navigate the seas.
The accident - the fourth in Asia involving a US warship this year - has led some to wonder about the US Navy's ability to act as the region's protector. The collision also caused confusion among some when Singapore and Malaysia both claimed that it took place in their respective waters.
HOW DOES SINGAPORE TRACK VESSELS?
To keep watch over Singapore waters, the MPA operates the VTIS, which monitors ships by means of radar as well as vessels' Automatic Identification System (AIS) signals. If ships are too close, for example, alerts would be sent to the vessels so that sailors can take necessary action to avoid a collision.
Singapore's VTIS, which it relies on to ensure safe navigation for all vessels, is touted as one of the most advanced and sophisticated.
While it is not known how vast the VTIS' monitoring area is, Mr Albert Lee, a former squadron commander of the Republic of Singapore Navy, said it would cover Singapore's territorial waters "to ensure safe navigation and security for all ships".
Analysts added that the Singapore Navy and the Police Coast Guard run their own maritime monitoring systems, though it is unclear if they detected the John S. McCain on Monday.
WHY DIDN'T THE VTIS DETECT THE AMERICAN WARSHIP?
Military experts said there is a high chance the US warship did not have its AIS activated, a common security practice among military ships. While international convention requires mandatory carriage of AIS for all commercial vessels, the rule does not apply to troop ships and warships.
7TH FLEET - THE LARGEST OF THE U.S. NAVY'S FORWARD DEPLOYED FLEETS
Number of ships.
Number of aircraft.
Navy and Marine Corps personnel assigned to the fleet.
Area of operation in sq km, including the west of the Pacific Ocean. It stretches from Kuril Islands in the north to the Antarctic in the south. Its headquarters are located at US Fleet Activities Yokosuka, in Yokosuka, Japan.
Year it was established. The fleet has maintained a continuous forward presence here, providing security and stability to the region for more than 70 years.
Vice-Admiral Phil Sawyer, following the dismissal of Vice-Adm Joseph Aucoin on Wednesday (Singapore time). The 7th Fleet is part of the US Pacific Fleet, which comes under the US Pacific Command.
Singapore's prime ministers have emphasised the 7th Fleet's importance for maritime security in the region. In the keynote address at the 2015 IISS Shangri-La Dialogue, Prime Minister Lee Hsien Loong said the US Pacific Command and the 7th Fleet are "a key factor for peace and stability in the region".
Lim Min Zhang
If the US warship's AIS was off, crucial information such as its location, identity, speed and direction of travel - to other vessels in the area and monitoring stations - would not have been available to Alnic MC and the MPA.
But this does not mean the warship did not know the tanker was there. When a vessel turns off its AIS, it can continue to receive information transmitted by other vessels. In this case, the MPA said it had detected the oil tanker, which means the Alnic had its AIS activated.
One theory that has surfaced is that the John S. McCain was a victim of a cyber attack affecting its systems and navigation. A CNN report suggested that the warship lost steering control before the collision.
Mr Ridzwan Rahmat, senior defence and security analyst at Jane's by IHS Markit, said it is also possible for warships to transmit jamming signals that can affect the accuracy of another ship's radar.
CAN A WARSHIP OPERATE COMPLETELY IN STEALTH MODE?
It is unclear if the tanker detected the John S. McCain, but naval experts said it is tough for military warships to operate completely in stealth mode at sea, even though they could switch off their AIS and take steps to weaken radar or sonar detection by others.
To detect the presence of other ships and objects, monitoring stations and vessels send out pulses of radio waves, which are reflected off the object back to the source.
To hide, while still being able to closely monitor other ships, military vessels are designed to reduce the radar cross-section (RCS), which is a measure of how detectable an object is with radar.
Features that can reduce the RCS include absorbent paint and surfaces angled to reflect signals elsewhere and away from the source.
Said a former Singapore naval officer: "With stealth technology, you can be a huge warship but appear on radar screens as a sampan. The whole point is to move under a cloak of secrecy."
But Mr Richard Bitzinger, a military expert at the S. Rajaratnam School of International Studies, pointed out that the class of destroyers that the US warship belongs to dates back to the 1980s and has limited stealth features. "It would appear on other ships' radar, even if its AIS and other transponders were switched off," he said.
WHY IS IT SO DIFFICULT TO RECOVER BODIES IN THE VESSEL?
Meanwhile, questions have been raised on why so few bodies have been recovered. Experts said the work is "delicate", can be done only by trained professionals and in most cases, cannot start immediately after a collision.
This is because damage control efforts must first be taken and checks done to ensure the ship is stable, said Mr Richard Tan, a former commanding officer of the Naval Diving Unit in the Singapore Navy.
How badly and where the ship was damaged can also add to the difficulty. In this case, divers needed to gain access to sealed compartments that were flooded to get to the sailors trapped inside, said Mr Kevin Loh, director at commercial diving company Dive Squad.
Mr Loh said an important consideration is the value of the vessel and whether further damage should be minimised.
IN WHOSE WATERS DID THE COLLISION TAKE PLACE?
The location of the collision - in waters near Pedra Branca, an islet at the eastern entrance to the Singapore Strait - has resurfaced an age-old territorial dispute between Singapore and Malaysia.
Both have historically laid claim to Pedra Branca, and both also claimed the collision took place in their territorial waters, which extend 12 nautical miles from shore, according to the United Nations Convention on the Law of the Sea.
Singapore released coordinates of the collision site to back its claim.
Soon after the accident, Malaysia's navy chief tweeted a map showing that the collision had taken place within Singapore's territorial waters, although the post now sports a new map.
Both sides also launched search-and-rescue efforts, although Singapore was reportedly the first to do so, deploying its ships and other resources to the accident site.
This included a helicopter that flew the injured sailors to the Singapore General Hospital.
Singapore's claim is based on a 2008 judgment by the International Court of Justice (ICJ) which opined that sovereignty over Pedra Branca belonged to Singapore.
Analysts said the claims and efforts of both countries must be seen in the light of the ICJ ruling, especially after Malaysia's actions this year in applying for a revision and interpretation of the ICJ ruling.
ISEAS - Yusof Ishak Institute research fellow Mustafa Izzuddin noted: "Had either Malaysia or Singapore not offered to lead the search-and-rescue mission, the country which did could then decide to make use of this incident to buttress its claim to the territorial waters in and around Pedra Branca."
SHOULD THE COLLISION RAISE CONCERNS ABOUT THE AMERICAN NAVY'S ABILITY?
Yet another collision involving an American warship this year has led some to voice concerns about the US Navy's operational integrity and, in turn, its ability to play a stabilising role in the region.
Maritime expert Collin Koh of the S. Rajaratnam School of International Studies said the recent incidents could affect the navy's operations.
The USS Fitzgerald - which was involved in an earlier accident that killed seven US sailors - is still out of action and now the John S. McCain is down too, he said. Both are "workhorses" of the US Navy in this region, he added.
But Mr Bitzinger said the bigger challenge facing the US Navy is one of fatigue and complacency.
Taking a more positive view, National University of Singapore's Associate Professor Bilveer Singh, who specialises in international relations, noted that the US 7th Fleet has nearly 70 ships.
"For US allies such as Japan and South Korea, the deterrent role and value of the 7th Fleet is not in any way affected as there is an air, sea and land forces component, not to mention nuclear weapons as well as troops in this region," he said.
• Additional reporting by Lin Yangchen