BERLIN (AP) — German authorities said Tuesday they suspect a male nurse of killing six elderly patients, and attempting to kill at least three more, after he admitted to injecting them with diabetes medication.

Munich police said the 36-year-old Polish man has acknowledged injecting 12 people in his care with unnecessary insulin shots from April 2017 until his arrest in February this year.

The suspect, previously identified as Grzegorz Stanislaw Wolsztajn, denies intending to kill them, police said. He is also being charged with three cases of serious bodily harm.

Wolsztajn worked for various Poland and Slovakia-based temp agencies in at least 68 locations across Germany since 2015, often staying only for short periods of time.

The suspect is alleged to have sought out vulnerable patients in order to steal their belongings. When he failed to find valuables he would quit his job and seek a new position elsewhere.

Police believe further crimes were averted by the man's arrest in February on suspicion of one killing, saying: "The accused already had new positions lined up."

German authorities initially reached out to police in England amid concerns that Wolsztajn may also have committed crimes there. But a spokesman for Munich police, Florian Hirschauer, said investigators determined this wasn't the case.

The suspect is believed to have started injecting the patients, aged between 66 and 91, with insulin only after he himself was prescribed the diabetes drug and informed of the risk of overdoses in January 2017, police said.

The case echoes that of a former hospital nurse, Niels Hoegel, accused of killing 100 patients at two hospitals in Germany over a decade ago. The 41-year-old, who is already serving a life sentence for murder, went on trial in the northwestern city of Oldenburg last month.