Health bosses have admitted they may never be able to find out how a premature baby became fatally infected with E.coli.
Two babies died at Singleton hospital in Swansea after it was discovered they had the bacteria in their bloodstream.
It is believed officials know how one of the tots had become infected with ESBL E.coli. They would not comment on the issue, due to strict confidentiality rules, but said they would not be conducting any further investigations into that death.
However, they are continuing their enquiries into the case of a second baby - named locally as Hope Erin Evans - and an unrelated woman, who tested positive for the bug but has not presented any symptoms. So far, bosses have drawn a blank on how cross-infection might have occurred.
A spokeswoman for Abertawe Bro Morgannwg University Health Board (ABM), which runs Singleton hospital, said: "Our investigations into the cross-infection of the two cases have not yet shown any evidence of how the bug was transmitted to the baby or the woman, as we have not found any evidence of ESBL E.coli within the environment.
"We appreciate this is frustrating and concerning for the individuals involved. If any further evidence emerges that confirms the cause of the transmission then the families will be informed immediately. However, in other similar instances of this kind elsewhere in the UK the cause of transmission was never identified."
In most people, ESBL E.coli is not harmful but in vulnerable individuals, such as premature babies and the elderly, it can cause serious infections.
So far, five cases of ESBL E.coli have been confirmed by ABM. Three of these - one which saw a young baby die - were later found to have been contracted from outside Singleton Hospital and are not under investigation.
ABM said the two remaining cases appeared to be "isolated incidents which have been contained" and there was no evidence of the infection spreading further.
Officials said they hoped to fully reopen the neonatal unit at Singleton Hospital very shortly, and wanted to reassure women the maternity unit remained open as normal for full-term births. The board also said any mothers-to-be who have concerns or questions should speak to their community midwife in the first place. In addition, people can also telephone a help-line service on 07747 615627.