Oil tanker Alnic MC was detected by the Singapore maritime authorities before its collision with American destroyer USS John S. McCain in Singapore waters on Monday morning.
The Maritime and Port Authority of Singapore (MPA) said in response to queries from The Straits Times yesterday that its Vessel Traffic Information System (VTIS) had detected only the presence of Alnic MC.
The monitoring system detects and tracks vessels via radar or the Automatic Identification System (AIS) signals, and enables an alert to be sent to vessels in the event of an impending collision and to get them to change course.
The MPA said the International Convention for the Safety of Life at Sea requiring mandatory carriage of AIS is, however, not applicable to ships of war and troop ships.
"In this case, MPA's VTIS and radar detected only the presence of the tanker," it said when asked by ST whether it had tracked the two vessels before the collision.
"The incident occurred in Singapore waters in the Traffic Separation Scheme in the Singapore Strait. Investigations are ongoing."
Naval experts say the VTIS had likely detected the tanker through the latter's AIS. But the AIS in the warship might have been switched off. Still, other vessels in the vicinity should have been able to detect or see the destroyer on their radar systems or by sight.
Even as a probe is under way, experts have also suggested other possible causes for the collision - including a potential cyber breach that may have disabled the destroyer's computer system. It has also been suggested that the warship could have been hit by a technical failure that jammed its steering. Human error is another possible cause that has been floated.
Admiral Scott Swift, commander of the US Pacific Fleet, told reporters in Singapore on Tuesday that nothing is being ruled out in the investigations. This includes possible negligence on the part of the crew, which will be investigated, he said, adding that "every scenario will be reviewed and investigated in detail".
The collision left 10 US sailors missing and five injured. The US Navy said on Tuesday that the bodies of some sailors had been found.
Military experts say it is not uncommon for naval vessels to switch off their AIS systems for security purposes. Dr Sam Bateman, a former commodore of the Royal Australian Navy and adviser to the Maritime Security Programme at the S. Rajaratnam School of International Studies, said: "Warships often don't have their AIS switched on because of security... That is a real problem."
The International Maritime Organisation - a United Nations agency that oversees the safety and security of the global shipping sector - told ST it is compulsory for all merchant ships to switch on their AIS and other detection systems. But the guidelines do not apply to military vessels.
"While not mandatory, we do encourage all vessels - military and commercial - to adhere to the guidelines," a spokesman said, pointing out that cyber attacks against the shipping industry are a growing concern.
•Additional reporting by Zaihan Mohamed Yusof