Lucy Letby, 34, was found guilty of murdering seven infants and attempting to murder six others between June 2015 and June 2016

A senior doctor told a public inquiry that British nurse Lucy Letby likely murdered or assaulted more children than she was convicted of killing.

"On reflection I think it's likely that Letby didn't start becoming a killer in June 2015, or didn't start harming babies in June 2015," Stephen Brearey, a consultant pediatrician, told the public inquiry, which is looking into events surrounding the deaths, the BBC reported. "I think it's likely that her actions prior to then over a period of time changed what we perceived to be abnormal."

Letby, 34, was found guilty of murdering seven infants and attempting to murder six others between June 2015 and June 2016 while working as a neonatal nurse at Chester Hospital in Cheshire, England.

The seven deaths were caused by being given excess milk, air, insulin or fluid, prosecutors have said.

Related: Why Will Killer Nurse Lucy Letby Go Back to Court Tomorrow for Another Trial?

The former nurse was sentenced to life in prison and is considered one of the most prolific child killers in United Kingdom history.

During the public inquiry chaired by High Court judge Lady Justice Thirlwall, Brearey said he didn't know of anyone who was suspicious of Letby prior to June 2015, but there now appeared to be suspicious cases, according to the Times of London.

"If we had a thermostat for the level of work and number of events that we can't quite understand, I think it was turned up over those years," he said, per the Times. "So our perception of what is normal for a neonatal unit in terms of number of collapses you might expect in a week, a month, or a year had changed."

Related: Lucy Letby Trial: What to Know About the Nurse Convicted of Murdering 7 Infants in U.K. Hospital

Brearey was among the staff members who raised suspicions about Letby at Chester Hospital. One senior doctor, Dr. Ravi Jayaram, testified during Letby's trial that he observed her standing over an infant whose breathing tube was dislodged, watching the baby's oxygen levels dropping and doing nothing. The doctor intervened, but the baby died three days later.

The Cheshire Constabulary launched its own investigation into the deaths in May 2017, after they were alerted to Letby's suspicious behavior

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During her trial, Letby took the stand in her own defense, telling the jury that she meant no harm during her time working at the Countess of Chester Hospital. "I only ever did my best to care for them," Letby said of the babies in her care. "That's completely against everything that being a nurse is. I am there to care not to harm."

Letby claimed in court that the infant deaths could be related to alleged plumbing issues in the hospital.