UNITED NEWS INTERNATIONAL (UNI) — A German nurse is charged with 97 counts of murder.

The 41-year-old suspect, Niels Högel, is already serving a life sentence for killing patients. However, during his previous trial in 2015, he confessed to killing more; pushing police to investigate further.

On Jan. 22, Högel appeared in court again and fresh charges were filed.

Högel allegedly injected his patients at hospitals in Delmenhorst and in Oldenburg, with a drug that can cause heart failure. He previously testified that he acted out of boredom.

Police have exhumed bodies buried over the last two decades and reviewed patients files to gather evidence.

The police say local authorities also failed to do their job, by not reporting an increase in fatalities from when Högel was working.

If Högel is found guilty of 97 murders, it will make him Germany’s worst killer since World War II.