LONDON — A health care worker has been arrested on suspicion of killing eight babies and attempting to kill six others at a hospital in northern England, the police said on Tuesday.

The arrest came after a yearlong investigation by the Cheshire Constabulary that has grown to include the deaths of 17 infants and what the police called “15 nonfatal collapses” of babies at the Countess of Chester Hospital’s neonatal unit between March 2015 and July 2016.

“This is a highly complex and very sensitive investigation, and, as you can appreciate, we need to ensure we do everything we possibly can to try to establish in detail what has led to these baby deaths and collapses,” Detective Inspector Paul Hughes said in a statement.

Detectives started the investigation in May 2017, after the hospital reported a high mortality rate at the unit that it could not explain.