Authorities say 4-year-old Wilhelm Ducatl was taken to the hospital with stomach pains back in May and died two days later.
Detectives realized that three months earlier, his 63-year-old grandmother, Tofoon Man, died from what appeared to be the same symptoms.
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The Medical Examiner's Officer did post mortem examinations on both, exhuming the grandmother's remains.
The cause of death for both individuals was determined to be acute thallium poisoning, and both were ruled homicides.
Thallium is a lethal metal used in the manufacture of electronics.
Both poisonings may have occurred at the family home in Brooklyn, on 65th Street in Bensonhurst. Each time, medics raced to the apartment with doctors later unable to save them.
Detectives are now interviewing the parents, other family members, and anyone else who may be involved, but the remains a mystery.
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The names of the surviving family members could not be confirmed, and they could not be reached for comment.
There are no arrests at this time, and the investigation remains ongoing.
Anyone with information is asked to call the NYPD's Crime Stoppers Hotline at 1-800-577-TIPS (8477) or for Spanish, 1-888-57-PISTA (74782).
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