The Bugis Junction cluster may have stemmed from an infected visitor, from whom the virus then spread among employees at the mall, said Singapore's director of medical services Kenneth Mak yesterday.
He added that the spread among employees in the mall could be due to a lack of compliance with safe management measures, a situation that has been observed at other clusters as well.
"We're still investigating the clusters and examining how the spread may have occurred, but we think that it may have come from outside the mall," he said at a press conference held by the multi-ministry task force on Covid-19.
"But then as a result of spread within the workplace, then others were infected, involving the department store within the mall and other outlets," he added.
The cluster at Bugis Junction emerged on Aug 24 with 20 cases, and ballooned to 254 cases as at yesterday. The first 20 cases were all mall staff.
Yesterday morning, those working at the mall said staff at department store BHG accounted for at least 70 cases in the cluster.
Prof Mak said: "This is a common situation that we've noticed happening in other clusters as well, where it starts off with the introduction from the community coming in and then the workplace if there is insufficient discipline in complying with safe management measures."
He added that those infected within the mall could have sparked the transmission through mask-off activities such as eating, or while using common facilities such as the toilets.
Trade and Industry Minister Gan Kim Yong, co-chair of the multi-ministry task force, added that it is extremely difficult to trace back to who started the transmission in the mall.
"We just have to look at the setting and assess what is the most likely (route) the virus had gone through.
"Probably one of the customers or employees got infected and it spread among the employees in the mall, in the department store, and it subsequently passed on to other customers, and then it became a bigger cluster."
Health Minister Ong Ye Kung, co-chair of the task force, said although the Bugis Junction cluster has grown, the number of new infections each day is being suppressed with timely contact tracing efforts.
He said: "We went in, we closed the store, we contact-traced, tested everybody, quarantined, and today that cluster, after a week or so, pretty much has been suppressed. Now we're at the tail end (of the cluster)."
For instance, at the start of the Bugis Junction cluster, the number of cases linked to it rose from 20 cases on Aug 24 to 61 cases on Aug 25. But the number of such cases went down to 13 on Thursday and eight yesterday.
Mr Ong, however, said the strategy to contain the infections in the bus interchanges has to be different as public transport is an essential service. "If we close down interchanges, the whole system gets paralysed. So we are taking a longer period to trace, test, isolate, without closing down and disrupting essential services," he added.