A German nurse was convicted of murdering two patients. Police say he killed 97 more

Former nurse Niels Hoegel, accused of multiple murder and attempted murder of patients, pictured in 2015 covering his face with a file at a court in Oldenburg, Germany. Photo / AP file

Niels Hoegel's deadly drugs of choice were ajmaline, lidocaine and calcium chloride - medications that he would administer to send his patients into arrhythmia, or force their blood pressure to crash to dangerously low levels.

Then, the former nurse testified, after he'd taken patients to the brink of death, he would revel in attempting to bring them back to life.

Except when he didn't.

Now, Germany is coming to terms with the full scope of one of its most prolific serial killers since World War II - an apparent mass murderer who managed to escape suspicion for more than a decade because his victims were already critically sick in intensive care units.

Hoegel was sentenced to life in prison after being convicted in 2015 of two murders and two attempted murders. He had previously been convicted of attempted murder in 2008, according to the BBC.

But those trials barely covered a fraction of what authorities now say was a murderous ritual repeated during a years-long killing spree with a staggering body count.

A new indictment charges Hoegel with killing an additional 97 patients at two different hospitals over half a decade.

Hoegel will face a new trial over an additional 97 deaths, and while Germany doesn't allow consecutive life sentences, the convictions could affect his chances of being released on parole.

The scope of his crimes first came to light in testimony at Hoegel's 2015 trial.

He said he had intentionally brought about cardiac distress in 90 patients at one facility where he had worked, and had killed people at another. He had made the same claims while bragging to other inmates.

The exact number can never be known because some of the people who died under Hoegel's watch were cremated, AP reported.

But authorities have exhumed 134 bodies from 67 cemeteries in three countries, looking for traces of the deadly drugs - and investigators have been able to sketch out a rough timeline of what they say is a string of serial killings that stretched across five years.

Police believe Hoegel killed his first victim in early 2000, when he was employed at a clinic in Oldenburg, close to the Dutch border, according to the Guardian. He worked there from 1999 to 2002.

After killing nearly 40 other patients, according to police, he moved to a hospital in Delmenhorst, near the northwestern city of Bremen, in 2003.

A week after starting the new job, he had killed again, investigators say.

He worked at that clinic for another two years before finally being discovered.

His first conviction came in 2008, but over the next decade came new revelations - and a rising body count.

The murders could have been prevented

Johann Kühme

Hoegel said the decision to administer life-threatening dosages of medication was "usually spontaneous." Each time someone died, according to the BBC, Hoegel vowed to never act again.

But afterward, his resolve would slowly fade.

He claimed to be "honestly sorry" for the deaths, and said in 2015 that he hoped his conviction would bring some peace to his victims' families.

Authorities laid some of the blame at the feet of officials at the hospitals where Hoegel worked, saying they could have acted faster to prevent deaths.

"The murders could have been prevented," said Johann Kühme, head of police in Oldenburg, where Hoegel is first thought to have killed. "People at the clinic in Oldenburg knew of the abnormalities."

Instead, Hoegel moved from hospital to hospital with a spotless record.

Authorities' reluctance to act apparently endured until the end of Hoegel's slayings, investigators said: In 2005, a colleague noticed him administering ajmaline into a patient, who died the next day.

The colleague reported Hoegel to hospital managers, who for some reason decided not to call the police or confront him, authorities said.

By the time they did, he had killed another patient - his last.