There's never been an explanation, and I don't think the question was asked at either the trial or Thirlwall. However, when Dr McPartland was asked to review the deaths in 2017, she pointed out that toxicology had not been ordered and it was too late to do it now.
We know that the children had post-mortems, and that the consultants would only have had to express concern to have a forensic post-mortem.
We know that the post-mortem form for baby O did not inform the coroner's office that the child's blood tests showed he bled dramatically after Dr Brearey performed an unusual and risky procedure. The pathologist doesn't review all medical notes though they can request more info. They work on the summary the hospital sends them.
We don't know what information was on baby P's form, but we know that both Dr Evans and Dr Bohin at the trial criticised aspects of his resuscitation.
We know that Dr McPartland, acting from the full medical notes which the pathologist did not have, sent back an explanation of the two deaths which categorised them as involving:
1a. Significant suboptimal care that is possibly relevant to the outcome,
1b. Failures in care to recognise problems and
1c. A failure to act appropriately
We know that baby O's notes showed that no consultant had attended his collapse and resuscitation for 40 minutes, when it was supposed to be five.
And we know that Shoo Lee's panel considered that baby O's death was caused by shock and haemorrhage after a liver hematoma was ruptured and went unrecognised by the treating team, possibly exacerbated by Dr Brearey's operation.
We know that the panel said baby P's death was caused by the team's poor management of his condition. Both children were affected by ventilator pressures that were too high, and baby P also received a higher than intended dose of adrenaline.
So - I don't think the doctors were plotting to blame Lucy Letby for these events and have her locked up. But I don't think they were committed to a full and open analysis of what went wrong for those two children either.