The incident is being described as a tragic accident.

BOSTON (AP) – Authorities were investigating Tuesday how two young children were given arsenic instead of water at a weekend barbecue, killing a baby boy and sending his sister to the hospital.

The Glynn family of Middleton was at a cookout Saturday in Nahant, a seaside town northeast of Boston, when host Constantine Pitsas poured what he said was water, but was actually the liquid poison, into the baby’s formula bottle, according to a statement from the family.

“Mr. Pitsas poured a substance, which he called spring water, into the formula bottle for our 4-month-old Benjamin and the drinking glass of his 2-year-old sister,” the Glynns said in the statement, which was released Tuesday by Essex District Attorney Jonathan Blodgett’s office. “This substance was later identified to be arsenic.”

The family’s statement said the girl was recovering.

Pitsas did not return a message left at his home. The phone listing for the Glynn family was disconnected.

The poisoning was characterized as a tragic accident by police and state officials, but nonetheless sparked an investigation from Blodgett’s office and the state’s child welfare agency.

“We’re continuing to collect evidence, and we’re continuing to conduct interviews, and the investigative process remains ongoing,” said Steve O’Connell, a spokesman for Blodgett.

The arsenic was kept in a jug stored with similar containers of water, according to police.

After the children drank the arsenic, they immediately fell ill, and their parents took them to a Beverly hospital, then later to Children’s Hospital, the regional poison control center.

The discovery that the liquid was arsenic sparked a high-speed interstate relay on Sunday to bring an antidote 240 miles from the same Bangor, Maine, hospital that treated members of a church congregation who drank arsenic-laced coffee at their church in April. The serum arrived too late to save Benjamin, who died Monday.

The girl remained at Children’s in Boston on Tuesday in fair condition, according to hospital spokeswoman Andrea Duggan.

An autopsy was performed Tuesday on Benjamin, but O’Connell said investigators would confer Wednesday morning before making the results public.

The state Department of Public Health determined that there were “very high levels” of arsenic in the formula that Benjamin drank. Arsenic is used in rat poison, pesticides and wood preservation. The effects of poisoning vary, including sore throat, nausea, vomiting and death.

The state Department of Social Services, which on Monday evening received a report of child neglect required by state child protection laws, was investigating the case.

“Our job is to make sure that homes are safe for children, and that there’s no child endangerment. That’s something that we’d investigate right alongside the police,” said DSS spokeswoman Denise Monteiro.

AP-ES-08-12-03 1925EDT