A woman held on suspicion of the murder of eight babies and the attempted murder of another six at a hospital is 28-year-old nurse Lucy Letby.
Ms Letby was arrested on Tuesday in a probe into the deaths of 17 infants at the Countess of Chester Hospital.
Police are searching her house in Westbourne Road, Chester, about a mile from the hospital.
Detectives are also investigating 15 non-fatal collapses at the neonatal unit between March 2015 and July 2016.
Officers were also seen at Ms Letby's parents' property in Hereford.
The arrest comes as part of a long-running investigation into a higher than expected number of baby deaths at the hospital.
The cases being investigated involve families from the Cheshire and north Wales area.
Cheshire Police has not officially named the suspect but said the woman arrested remains in custody.
Police are said to have arrived at the home in Chester at about 06:00 BST on Tuesday.
A neighbour said: "I saw lots of police cars arrive, a few cars then a big police van, and gradually more and more came and they put up the tent."
Local residents said officers moved a small, silver Suzuki car, which had an NHS parking permit in the window, from the driveway of the address to erect the tent.
In a 2013 interview with the Chester And District Standard, Ms Letby said she began working on the unit after graduating and cared for babies requiring various levels of support.
She had worked on the unit as a student nurse during three years of training before qualifying as a children's nurse at The University of Chester in 2011.
The Countess of Chester Hospital declined to comment on whether any staff had been suspended.
Liverpool Women's NHS Foundation Trust also confirmed Ms Letby did training placements with them.
A Trust spokesperson said: "We are co-operating with police as part of their investigation which includes a routine review of patients cared for on our Neonatal Unit during the time of these placements.
"There is currently no suggestion that any patients at Liverpool Women's came to any harm in relation to this investigation."
Police were asked to investigate the deaths of eight babies at Chester in May 2017, a probe which has since widened to include nine more.