Prosecutors said they received more reports of suspicious deaths after two male nurses admitted over the weekend to killing 16 patients at two hospitals in Uruguay.
Officials would not say how many deaths are under review, but have said that one of the suspects admitted killing 11 patients, and the other five.
The daily El Pais reported that the death toll could exceed 50.
"More complaints are arriving and we will be working specifically on them," to determine if there were more victims, said Angel Rosas, a federal official leading the investigation.
Police arrested the two male nurses on Sunday and charged them with murdering patients at a private Neurological Intensive Care Center and a public hospital's intermediate-level care unit.
The names of the suspects have not been released, but the El Observador newspaper identified them as Ariel Acevedo, 46, and Marcelo Pereira, 40.
The men confessed to the killings, telling investigators "they didn't like seeing people suffer," said Rolando Vomero, the judge overseeing the investigation.
One of the men's girlfriend, who also is a nurse, was arrested and charged with covering up information in the case.
Health Minister Jorge Venegas described the suspects as "serial killers," and promised far-reaching inspections of both private and public medical facilities, in consultation with the World Health Organization.
Venegas also said he would open a confidential hotline for tipsters to report suspected abuse, and a telephone counseling service to help those emotionally burdened by the scandal, unprecedented in this nation of three million.
It was "painful and disgusting" to contemplate "the presence of murderers" within the country's healthcare industry, said Deputy Health Minister Leonel Briozzo.
"There was no error here," Briozzo said. "There was intent to kill."
Ines Mazziotti, who represents one of the suspects, said that her client's acts were not homicides but rather "mercy" killings intended to put desperately ill patients out of their misery.
"After 20 years in intensive care, of constant stress on the edge of life and death, he could not take it anymore," Mazziotti said.
Rosas was unimpressed. "For us this is criminal conduct," he said. "The pity hypothesis collapses" after a patient that was being released died after receiving an injection, he said.
The case became known to authorities in January following a tip from "private sources." Authorities stepped up their investigation following the suspicious death of a female patient last week.
Vomero told reporters on Sunday that the alleged killers used different methods to end the lives of their patients.
In one case "the suspect introduced morphine directly into the blood stream," Vomero said, while in the other, "they pumped air into the blood stream, which resulted in deaths in just a few minutes."
The staff at Hospital Maciel, in historic downtown Montevideo, was trying to come to terms with having worked for years with one of the alleged killers.
"There is no doubt that his job performance was mediocre, he was bad with the patients and ... many of us distrusted him," a female nurse at the hospital told AFP, speaking on condition of anonymity.
"Anyone able to do this does not deserve to be called a human," she sobbed.