The child, identified as Anthony Delgado, suffered the cardiac condition in his apartment building on Steuben Street in the Park Hill section and was rushed to Staten Island University Hospital North, where he was pronounced dead.
"She was a very loving mother to her son," neighbor Sherri Sherwood said. "I can't say anything bad about her. It's a shock."
"Happy baby, good baby, really cute baby, the cutest baby ever," said Krystal Robles, a family friend.
Dozens remembered 16-month-old Anthony Delgado at a candlelight vigil outside his Staten Island home.
It was an emotional night for friends, family, and neighbors in the Park Hill section, many praying for his grieving father and mother.
"He meant everything to her. He meant everything to Marta. Her life revolved around that baby," said Lisa Floyd, a family friend.
Eyewitness News learned hospital doctors observed possible signs of trauma on the baby's body.
Police say the 31-year-old baby sitter, believed to be a resident of the same apartment building, had been taking care of the child for two days.
"I seen the baby yesterday with somebody, one of the neighbors, yesterday," neighbor Martha Charles said. "They never leave that baby with nobody."
The boy's grandmother found him unconscious and unresponsive in his crib shortly after 10 p.m.
Police are questioning his 31-year-old baby sitter.
They're also talking to her boyfriend.
"We put together a timeline. We have found that the babysitter took the child with her boyfriend to Manahttan and we are getting video of exactly her whereabouts," Det. Robert Boyce, Chief of Detectives, NYPD.
The baby sitter has 9 prior arrests.
Her 53-year-old boyfriend has 25 prior arrests.
Investigators believe the mother used the babysitter to watch little Anthony at least six times.
A memorial has been set up outside the apartment building:
"They are a very good family," Sherwood said. "We just can't understand. I just seen the baby the other day and he was all running around and everything."
The Medical Examiner will determine the cause of death. An autopsy is scheduled for Tuesday.
ACS released a statement saying: "We are investigating the circumstances that may have led to the heartbreaking and tragic death of this little boy."
"I pray they give them what they deserve because I believe in my heart that they did that, I believe that as a mother," Floyd said.