A German doctor will go on trial Monday for killing 15 patients with lethal injections, in what investigators fear may just be the tip of a deadly iceberg.

The 40-year-old palliative care specialist, named by German media as Johannes M., is accused of killing 12 women and three men between September 2021 and July 2024 while working in Berlin.

He allegedly injected the victims, aged between 25 and 94, with a deadly cocktail of sedatives and in some cases set fire to their homes in a bid to cover up his crimes.

A co-worker first raised the alarm about Johannes M. last July after becoming suspicious that so many of his patients had died in fires, according to Die Zeit newspaper.

He was arrested in August, with prosecutors initially linking him to four deaths.

But investigations threw up a host of other suspicious cases, and in April prosecutors charged Johannes M. with 15 counts of murder.

A further 96 cases are still being investigated, a prosecution spokesman told AFP, including the death of Johannes M.`s mother-in-law.

She had been suffering from cancer and mysteriously died the same weekend that Johannes M. and his wife went to visit her in Poland in early 2024, according to media reports.

- Muscle relaxant -

The suspect, dubbed "doctor death" by German media, reportedly trained as a radiologist and a general practitioner before going on to specialise in palliative care.

According to Die Zeit, he submitted a doctoral thesis in 2013 looking into the motives behind a series of killings in Frankfurt, which opened with the words "Why do people kill?"

Prosecutors say that in all 15 cases, Johannes M. "administered an anaesthetic and a muscle relaxant to his patients... without their knowledge or consent".

The relaxant "paralysed the respiratory muscles, leading to respiratory arrest and death within minutes".

In five cases, Johannes M. allegedly set fire to the victims` apartments after administering the injections.

On one occasion, he is accused of murdering two patients on the same day.

On the morning of July 8, 2024, he allegedly killed a 75-year-old man at his home in the Berlin district of Kreuzberg.

"A few hours later" he is said to have struck again, killing a 76-year-old woman in the neighbouring Neukoelln district.

Prosecutors say he started a fire in the woman`s apartment, but it went out.

"When he realised this, he allegedly informed a relative of the woman and claimed that he was standing in front of her flat and that nobody was answering the doorbell," prosecutors said.

In another case, Johannes M. "falsely claimed to have already begun resuscitation efforts" on a 56-year-old victim, who was initially kept alive by rescuers but died three days later in hospital.

- `No motive beyond killing` -

Johannes M. has not commented on the accusations against him.

Prosecutors say he had "no motive beyond killing" and are seeking a life sentence.

The case recalls that of notorious German nurse Niels Hoegel, who was handed a life sentence in 2019 for murdering 85 patients.

Hoegel, believed to be Germany`s most prolific serial killer, murdered hospital patients with lethal injections between 2000 and 2005, before he was eventually caught in the act.

More recently, a 27-year-old nurse was given a life sentence in 2023 for murdering two patients by deliberately administering unprescribed drugs.

In March, another nurse went on trial in Aachen accused of injecting 26 patients with large doses of sedatives or painkillers, resulting in nine deaths.

Last week, German police revealed they are investigating another doctor suspected of killing several mainly elderly patients.

Investigators are "reviewing" deaths linked to the doctor from the town of Pinneberg in northern Germany, just outside Hamburg, police and prosecutors said.