A legal expert has said that prosecutors have a weak case against the first-degree murder charge against Irish nanny Aisling Brady McCarthy, who they claim unleashed a vicious attack on the one-year-old Cambridge baby who was left in her care.

On Friday, Brady McCarthy, 34, was indicted on a murder charge. A Middlesex County grand jury also accused the nanny of assault and battery on a child.

Rehma Sabir died from severe head injuries in January, two days after her first birthday.

An expert in so-called "shaken baby" cases has said that the new murder charge would require prosecutors to prove premeditation and extreme cruelty.

“They’d need direct evidence to show that it was actually intentional, that it wasn’t just ‘the child slid out of my hands and dropped,’ ” said attorney Elaine Whitfield Sharp told the Boston Herald.  “They’d need proof beyond reasonable doubt that the force applied was applied intentionally.”

Sharp defended British au pair Louise Woodward, who was charged with first-degree murder in the 1997 death of 8-month-old Matthew Eappen in Newton. A jury convicted her of second-degree murder, but the conviction was reduced to involuntary manslaughter.

Sharp said the case against McCarthy contains “absolutely bucketloads of reasonable doubt.”

“In fact, I think reasonable doubt swallows the case,” Sharp said. “Just totally engulfs it.”

A spokeswoman for Middlesex District Attorney Gerard T. Leone would not comment on the evidence prosecutors plan to present but said: "“We feel confident in the case we’ll be putting forward."

The case against Aisling McCarthy Brady, charged with the murder of one-year-old Rehma Sabir in Boston, is beginning to show holes.Cambridge Day