[ArnoVW]

I think you're talking about Lucia de B. The first sentence of the article :

  When a Dutch nurse named Lucia de Berk...

In fact, she never confessed. As per her Wikipedia page:

  Important evidence at the appeal was to be the statement of a detainee in the Pieter Baan Center, a criminal psychological observation unit, where de Berk had said during outdoor exercise, "I released these 13 people from their suffering". However, during the appeal, the man withdrew his statement and stated that he had made it up.