A former nurse in Germany on Tuesday admitted to killing 100 people between February 2000 and June 2005 in two hospitals in Oldenburg and Delmenhorst, DW reported. Niels Högel is currently serving a life sentence for killing two patients.

Prosecutors said the former nurse would administer his victims with lethal injections so that he could impress his colleagues by trying to revive them. The prosecutors speculated that Högel committed the acts out of boredom.

The charges, which he has admitted to, make him one of the country’s deadliest post-war serial killers, BBC reported.

He hid his face behind a folder while being escorted into a festival hall that was serving as a courtroom to accommodate the 126 plaintiffs in Oldenburg.

Presiding judge Sebastian Bührmann said that the trial aimed at revealing the magnitude of the crimes. “We will do our utmost to learn the truth,” Bührmann said. “It is like a house with dark rooms – we want to bring light into the darkness.”

Högel was caught red-handed in Delmenhorst in the summer of 2005.

Authorities are also investigating why health officials did not report the crimes earlier. Investigators have exhumed 130 bodies for tests as part of the inquiry into the crimes.