The nurse targeted patients at the intensive care ward, Tuscany police said on Thursday.

None of the 13 victims in the Tuscan town of Piombino was terminal before her injections, according to the authorities.

The 55-year old mother of two allegedly injected the victims with lethal doses of anti-coagulant drugs, triggering multiple internal bleedings.

"We do not yet know of a specific motive," Carabinieri police commander Gennero Riccardi told reporters. "But the suspect suffers from depression," he added.

All of the victims were between 61 and 88 years of age, according to the authorities.

The police detained the veteran nurse after determining she was the only one on duty when all the deaths took place in 2014 and 2015.

'Killer nurse'

After her transfer from the intensive care unit in October, the mortality rate of the ward fell from 20 percent to 12 percent, according to Riccardi.

"In the horror rankings we have reached a new peak of human misery," Health Minister Beatrice Lorenzin said in a statement.

"Sadly this is not the first time that serial killings have been uncovered with a killer nurse as the protagonist," she added.

Several weeks ago, another Italian nurse was convicted to life in prison for killing a patient with a lethal injection of potassium. The 44-year old is still under investigation for around ten more suspicious deaths.

The former nurse also drew attention by allegedly taking "selfies" next to recently deceased patients.