A former colleague of a neonatal nurse who allegedly murdered multiple newborns told jurors this week that she thought "not again" when an infant suddenly collapsed after her twin brother had collapsed and died the night before.

Lucy Letby, 32, is standing trial in the U.K. for allegedly murdering seven babies and attempting to murder 10 more at the Countess of Chester Hospital’s neonatal unit between 2015 and 2016.

The prosecutor said she injected some infants with insulin or milk, while others she injected with air. She allegedly attempted to kill one baby three times.

Child A, whose family remains unidentified for legal reasons, was one of the babies Letby allegedly killed by injecting him with air when she came on shift June 8, 2015, just a little more than 24 hours after his premature birth, according to The Standard. Prosecutors allege she attempted to murder his twin sister, Child B, in the same way the following night.

ALLEGED KILLER NEONATAL NURSE LUCY LETBY WROTE CONFESSION NOTE: ‘I AM EVIL. I DID THIS.’

Neonatal nurse Lucy Letby

Neonatal nurse Lucy Letby, 32, is alleged to have murdered seven babies and attempted to kill 10 more. (Credit: SWNS)

Letby's coworker, who also remains unidentified, testified to the Manchester Crown Court on Monday that she had been preparing medicines when the alarm went off at Child B's incubator. The coworker recounted that Letby was the first to go to the child's cot and called for her help.

Regarding how Child B looked at the time, Letby's colleague said: "She looked very ill. She looked very like her brother did the night before. Pale, white, with this purple blotchy discoloration. It was all over her body."

Police search home of Lucy Letby

A police officer stands outside Lucy Letby's house in Chester, England, on July 4, 2018. (Christopher Furlong/Getty Images)

"I just remember thinking ‘not again’ — to see his sister with the same appearance," the colleague told the jury.

When they inserted a breathing tube into Child B, the child "started to stabilize quite quickly," the witness said, going on to add that Child A's "deterioration was very sudden and to an unusual degree. Babies can be very poorly quickly, but there is usually some indication that is happening. We had no undue concerns."

ALLEGED KILLER UK NEONATAL NURSE TRIAL: TWINS' MOTHER PLEADED FOR CHILD'S LIFE: ‘DON’T LET MY BABY DIE'

The Countess of Chester Hospital sign

The Countess of Chester Hospital in Chester, England, where nurse Lucy Letby used to work. (Christopher Furlong)

"To go from that is very unusual and then [Child B] had been good throughout the evening for me… then she became ill very quickly," said the witness. "She deteriorated very quickly and then this discoloration."

"You never want any baby to die. You want to help them go home to their families. That’s always been my goal," the witness added.

Lucy Letby (inset) and newborns

U.K. neonatal nurse Lucy Letby and a stock image of newborn babies (ER Productions Limited via Getty Images/SWNS)

Child B ultimately recovered and was discharged a month later, the court was told. The twins' mother had pleaded for the lives of her children as medics attempted to save them, according to recently heard court testimony.

Letby was a "constant, malevolent presence" in the neonatal unit of the hospital in northwestern England, prosecutor Nick Johnson argued before a jury when Letby's trial opened last month.

Johnson told jurors "a poisoner was at work" at the hospital, which he said had been marked by a "significant rise in the number of babies who were dying and in the number of serious catastrophic collapses" after January 2015, before which he said its rates of infant mortality were comparable to other busy hospitals.

Chester Police at Lucy Letby's house in 2018

A police officer leaves Lucy Letby's house in Chester, England, on July 3, 2018, after she was arrested on suspicion of murdering or attempting to murder babies. (Anthony Devlin/Getty Images)

Investigators found Letby was the "common denominator," and that the infant deaths aligned with her shifting work hours.

When searching her home in 2018, police found handwritten notes reading, "I am evil" and "I killed them on purpose because I'm not good enough," according to the prosecution.

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Letby pleaded not guilty to seven counts of murder and 15 counts of attempted murder. Her trial is slated to last for up to six months.