Dr. Virginia Soares de Souza wanted to free up hospital beds in the intensive care unit of the hospital where she worked, police authorities say.
So, they allege, she killed off patients to make room for new ones.
Brazil's health ministry alleges de Souza killed seven terminally ill patients at the Evangelical Hospital in the southern city of Curitiba in Brazil.
The health ministry press officer says the doctor and seven assistants are suspected of injecting the patients with "drug cocktails" and of tampering with their respirators. The press officer declined to be identified because she was not authorized to comment on the case.
In a wiretap recording, de Souza said: "I want to clear the intensive care unit. It's making me itch," according to the BBC.
"Unfortunately, our mission is to be go-betweens on the springboard to the next life," she added.
De Souza's attorney says that his client is being falsely accused by people who lack medical expertise.
"We will soon prove that everything that took place in that ICU [intensive care unit] is justified by the medical literature," said her lawyer, Elias Mattar Assad, according to the BBC.
Investigators suspect that there were far more killings – or intended killings – than the seven they've officially linked to de Souza.
Investigators are looking at the medical records of 300 other patients who were under de Souza’s care.
They did not provide details and did not say if they had died.
De Souza was arrested in Curitiba last month but released on bail a week ago pending the outcome of the investigation.
The doctor's actions drew the attention of authorities after hospital staff, including nurses and nursing technicians, reported their suspicion that she was causing the deaths of critically ill patients.
The Associated Press contributed to this report.