"Oh my God," she recalled. "The baby's 2 months old. What are you doing?"
The unidentified woman grew concerned watching Nicholas St. Francis, 23, with a group of friends in a reservoir in Spencer, Massachusetts — along with St. Francis' infant son.
"I said, 'What the heck is he doing?'" the neighbor, who didn't want to be identified, told NECN-TV in Newton. "I guess he dunked it because I really couldn't see all of it, and after that, everybody cheered. And the baby was crying."
Police said St. Francis was drinking with a group of friends when he dunked the baby in the Stiles Reservoir, across the street from St. Francis' house, adding that video was shot of St. Francis dipping the infant in the water, at one point submerging him.
"Nicholas was shown holding the infant and dipping him twice into the lake. The third time the entire child was submerged. The screaming child was lifted out of the water to cheers and laughter from the adults," Officer Valerie Morin wrote in a report, according to the Telegram & Gazette in Worcester.
St. Francis was arrested over the weekend on charges of assault and battery and reckless endangerment of a child, NECN noted, adding that he pleaded not guilty and was released Monday on $500 bail.
"Whether you were meaning it to be or not, it certainly is endangerment at that point," Spencer Police Sgt. John Agnew told NECN.
"You're on a lake, it's a large body of water, you have boats going by, you have waves going," Agnew added. "If the child slipped out of the hands, who knows what could've happened at that particular point."
Police, who said the baby's mother was at work Sunday, were alerted when a call from her friend who saw video of St. Francis dunking his son and appearing as if he was "trying to drown the baby," reported the Telegram & Gazette.
"She was concerned about the welfare, obviously, of the child at that time," Agnew told the station.
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According to court documents, the 2-month-old's parents were upset that police were investigating, and Ms. Ballou (the infant's mother) was angry that her friend had alerted police.
"Kendra stated she watched videos of baby dunking on YouTube. All the videos claim the baby could not drown. All you had to do was blow into the baby's face and the baby will start to breathe again," the report said.
Mr. St. Francis told police he'd been drinking but the baby was not harmed "so he did not understand what the problem was," according to court documents.
Court documents indicate the infant's parents brought him to a hospital as a precaution, noting to police they're good parents. The Massachusetts Department of Children and Families is investigating, NECN added.
"I don't approve of what he did," said the neighbor. "Hate to see that happen."