There have been no arrests in the case, the police said, but the deaths of Wilhelm Ducatl and Tofoon Man are being investigated as homicides.
When a 4-year-old boy, Wilhelm Ducatl, died of a strange stomach illness in Brooklyn last May, it appeared he had tragically passed of natural causes. But when a doctor conducted a post-mortem examination, he discovered a strange rash on the boy and ordered a toxicology exam.
Detectives who later canvassed the boy’s neighborhood then unearthed a surprising connection: Just months before, Wilhelm’s 63-year-old grandmother, Tofoon Man, had also died after being admitted to the hospital for strange stomach pains — and a similar rash.
Now, investigators say the similarities between the woman’s death and her grandson’s were not coincidental — and that they were both killed, most likely by a material found in rodent poisoning that may have been put in their food while at the boy’s home in the Bensonhurst neighborhood of Brooklyn.
After the city medical examiner’s office found that Wilhelm may have been poisoned, Ms. Man’s body was exhumed and investigators determined the two had not died of natural causes.
The police said that no one had been arrested in connection with the case as of Thursday morning. Ms. Man’s daughter, who is also Wilhelm’s mother, was interviewed by investigators twice last year in connection with the deaths, but has not been charged with a crime.
Earlier this winter, the city medical examiner’s office ruled the official cause of death for Ms. Man and her grandson to be acute thallium poisoning. The Police Department announced those findings on Thursday — exactly one year after Ms. Man’s death.
The police said that Ms. Man also held an address on the Lower East Side of Manhattan, but it was unclear whether she had been living full time at the Brooklyn home or was just visiting at the time of her death.
Outside the six-story apartment building in Brooklyn where Ms. Man and her grandson had fallen ill, television news crews gathered on the sidewalks Thursday afternoon as neighbors walked by. Some said they recalled police activity outside the building about a month ago.
Thallium was used as a material in rodent poisons through 1972, according to the Centers for Disease Control and Prevention, but has since been banned in the United States because accidental exposure can cause significant harm to people. The effects include stomach pain and vomiting, “followed by the failure of multiple body organs, brain injury, and death.”
Thallium, a heavy metal, is no longer produced in the United States, but is still imported to manufacture certain electronics and medical materials.
It can be spread through food contamination or released as particles into the air. And because it is tasteless and odorless, the C.D.C. says that it “has been used by murderers as a difficult to detect poison.”
Karen Zraick contributed reporting. Susan C. Beachy contributed research.
