Dubliner Luke Waters, who spent 20 years on the force in New York, told RTÉ Radio’s Seán O’Rourke: “I was very sceptical from the very, very beginning.”
He said his first concern was how long it took for a grand jury decision that Ms Brady McCarthy should be charged.
He said he investigated 150 homicides a year and on average, the grand jury process took five days, but Ms Brady McCarthy’s took three months.
He was also astounded that the charge laid against her was first degree murder, which requires proof that a killing was intentional.
“I have never, ever gone to trial with murder in the first degree and got a conviction for that because it must be clearly premeditated.”
He was also uneasy because the prosecution team previously attempted a first degree murder prosecution against British nanny Louise Woodward in a similar case in 1997 which proved flawed.
Mr Waters said the case against Ms Brady McCarthy should have been dismissed at least two years ago when it emerged that, having originally said that only they and Ms Brady McCarthy had access to baby Rehma Sabir, her parents had in fact taken their daughter to Pakistan and Saudi Arabia without the nanny and she had suffered a fall and head injury while away.
“Right there, that’s grounds for dismissal,” he said. “It’s a pity it didn’t happen sooner.”