Three months on, officials are no closer to solving the mystery behind the cluster of 13 Covid-19 cases that emerged at the Mandarin Orchard Singapore hotel at the end of last year.
After two weeks of investigations, with the help of experts from other fields, the Ministry of Health (MOH) has not been able to say how the virus had spread within this group.
Apart from a couple who had flown in from Britain, the rest did not know one another.
They had come to Singapore from 10 widely dispersed places: Bahrain, Canada, Indonesia, Myanmar, the Netherlands, Philippines, South Korea, United Arab Emirates, Britain and the United States.
But somehow, they were all part of the same Covid-19 cluster. The genetic sequence of the virus that caused their infection was too similar for it to be anything else.
None were diagnosed on arrival in Singapore and they were all serving their routine two weeks of isolation at the hotel.
It appears that at least 12 of them had been infected at the hotel with a virus one of them had brought in from overseas.
That is because no one else - the hotel staff or the other guests who were infected - had viruses that were so genetically similar.
So the virus must have been brought in by one of the 13 people who were on stay-home notice (SHN) at the hotel, and spread to the rest.
On Dec 19 last year, following preliminary investigations, the ministry announced: "We cannot exclude that transmission could have occurred at Mandarin Orchard Singapore."
But how? That question remains unanswered to this day.
MOH had called in experts from the Building and Construction Authority (BCA) and the National Environment Agency (NEA) to help with the investigations. The Singapore Tourism Board (STB) was also involved as it oversees hotels.
All guests, including local guests on staycation at the hotel's other wing, were moved out, and the hotel shut its doors during the investigation.
About 500 hotel staff were checked for Covid-19, in case any of them proved to be the link for the transmission, or might have been infected by one of the 13. That would have required contact tracing to ring-fence any spread.
More than 20 officers spent close to two weeks at the hotel looking for possible breaches - such as transmission due to human-to-human contact, as well as building-related risks due to weaknesses in the ventilation system.
The severe acute respiratory syndrome came to Singapore in 2003 via a woman who returned from Hong Kong, where she had picked up the virus in a hotel lift - which an infected person had used before her - most likely from touching a lift button.
Covid-19 is from the same family of coronavirus.
But the team looking into what happened at the Mandarin Orchard hotel remain stumped, although they had found three areas where the situation had "issues" and "gaps that need to be strengthened": the check-in process, the way testing for those on SHN was done, and the hotel's ventilation system.
Associate Professor Teoh Yee Leong, a senior consultant in the MOH's epidemiology team, Communicable Diseases Division, told The Straits Times: "However, none of these findings could on its own account for how all 13 cases were infected over a short period of time."
The easiest answer would have been if all 13 guests had arrived on the same day and checked in at the same time.
But this did not happen. They had arrived on different days and times, between Oct 22 and Oct 30.
This led Prof Teoh to conclude: "As these 13 cases did not arrive on the same day and timing, but instead over several days, this excluded one of the possibilities that they were infected at the same time en route to the hotel or during the check-in process."
Although their arrivals were over a nine-day period, could all 13 have spent some days together staying on the same floor of the hotel?
Again, the answer is no.
Five of the 13 people had stayed on the 27th storey. But the other eight were scattered over seven other floors - from the ninth storey to the 24th storey - of the hotel's main tower, dedicated to people serving their SHN.
Although they stayed on different floors, their room numbers - XX31-34 and XX43-45 - seem to indicate some clustering at one part of the corridor.
With 446 rooms spread over 28 floors of the wing, there are roughly 16 rooms per floor - with XX3X and XX4X on opposite ends of the corridor.
But neither the hotel nor the STB confirmed if this was the case.
For example, there were four people in rooms XX32 - which is room 32 on different floors - and three in XX33.
However, Prof Teoh said: "Our investigations did not point towards a specific risk regarding common floor or common sections of rooms as they were staying at different levels and sections."
None of them had breached their SHN to wander around and spread the disease.
One possibility of how the virus had spread was when they went to a central location at the hotel to test for the virus.
Prof Teoh said investigations had found some hotel workflow issues, including the polymerase chain reaction testing queue.
There were also issues with the ventilation system based on "the walk-through inspection and environmental sampling" by experts from the BCA and the NEA.
But he added that it is impossible to say if any of these, on their own or in combination with other lapses, "may be the cause for the spread as our investigations were done at a later time".
On the conclusion of the investigations, Prof Teoh said: "Our assessment suggests that infection protocols and ventilation systems can be improved, for example, through better crowd regulation as well as having good ventilation systems to circulate more fresh air."
As for who was the cluster's likely Patient Zero who brought the virus to Singapore, Prof Teoh said: "The first reported case of these 13 cases could have been the Patient Zero, but we are not able to conclude about the chain of transmission as these happened some time ago."
That would be the 30-year-old Indian work pass holder who had arrived from the United Arab Emirates. He was the first to be confirmed with Covid-19 on Nov 2.
Four days later, on Nov 6, five more people in the cluster were confirmed to have been infected.
They are two Singaporeans, one returning from Britain and the other from Canada; a permanent resident returning from South Korea; a work pass holder arriving from the Netherlands; and a woman allowed in from Britain to visit her child who is a PR here.
But one of them, the Singaporean who had returned from Britain, could as easily have been Patient Zero, as her symptoms emerged on Nov 2 - indicating that she had been infected prior to that date.
Nov 2 was also the date the first of the 13 people in the cluster was diagnosed.
But the Indian national had not been sick and was diagnosed from routine testing of those serving SHN.
The other seven were confirmed between Nov 8 and Nov 11 - and were unlikely to have brought the virus into the country.
The time between exposure and the onset of symptoms is usually five to six days, though outliers might take as long as a week.
The more important question is if there are similar lapses or shortcomings at other hotels offering SHN facilities.
When asked, Ms Geraldine Yeo, director of the SHN operations division of the STB, said it is working with the BCA and the NEA on stricter audits of ventilation systems at SHN facilities.
She added that the board is ensuring that more than 70 SHN facilities follow strict infection control measures. They have been advised to improve infection prevention protocols and ventilation systems.
"STB will not hesitate to take enforcement action for non-compliance by businesses and individuals," she said.
No action will be taken against Mandarin Orchard hotel as it had not breached any of the protocols.