The baby died Monday. The day before, state police raced 240 miles to Boston from a hospital in Bangor, Maine, that treated poisoning victims earlier this year. Bad weather had made an airlift impossible, the Essex district attorney's office said.
The infant's parents were at a private party Saturday when they asked for water for their two children. The poison was stored in a plastic jug similar to jugs filled with water, a law enforcement official told The Boston Globe.
"At this point in time, we are looking at this as a tragic accident," Nahant Police Chief William F. Waters said. He said their was no reason to believe that the party host was trying to harm the children.
Police are trying to determine why the homeowner had the arsenic and why he kept it with the water jugs, Waters said.
The children became increasingly ill and the parents brought them to the hospital, the prosecutor's office said. Officials at Children's Hospital sought the drug DMPS, which is not approved by the Food and Drug Administration but may be used to treat arsenic poisoning in extreme cases, a hospital spokeswoman said.
The medicine was located at the hospital in Bangor that treated people in April who drank coffee tainted with arsenic at their church.
Michelle Davis, the hospital spokeswoman, would not say whether the drug was administered to the baby before he died. She also declined to release the girl's condition.
Tests of the baby boy's formula showed "very high levels" of arsenic, said Department of Public Health spokeswoman Roseanne Pawelec.
An autopsy was planned for Tuesday.
No one else at the party became ill.
Household uses for arsenic include rat poison, pesticides and wood preservatives.