Almost three years after she was arrested following the death of a baby girl in Cambridge, Massachusetts, Aisling Brady McCarthy learned yesterday that the murder charge against her was being dropped.

Her lawyer said that the Co Cavan woman has not been able to stop crying with joy, but added that her life has been ruined as Ms Brady McCarthy has spent almost two-and-a-half years in jail and several months under house arrest for a crime she did not commit.

Washington Correspondent Caitriona Perry, who has been following this case, explains what’s happening now.

Aisling Brady McCarthy has been cleared of murder – but how was it that she came to be arrested in the first place?

She had been working as a Nanny for the previous six months for the Sabir family.  On the day in question – Baby Rehma’s first birthday – the court documents show that Ms Brady McCarthy and the child’s mother fed her blueberry porridge for breakfast.

Then they all played with the new toys she had received for her birthday – a toy train and a toy tea set and they were playing at drinking tea from the little cups, then as Ms Brady McCarthy and the baby continued to play, the mother stepped away and left for work so the baby wouldn’t notice her mother leaving and start to cry.

The day passed as normal, according to the documents. The baby’s paternal grandparents, who were in town for her birthday, stopped by to visit, as did another mother and baby who were “nanny sharing” the Cavan woman with the Sabir family.

Then at lunchtime the baby seemed tired and her eyes were only half open. Ms McCarthy Brady thought she was sleepy and put her to bed.

A few hours later she found the baby breathing but unconscious, and the alarm was raised.

Within 20 hours of arriving at the hospital a doctor diagnosed shaken baby syndrome and police began questioning Ms Brady McCarthy. The baby girl was declared brain dead two days later and two days after that life support was turned off. As the last person with the infant, Ms Brady McCarthy was ultimately arrested and charged with murdering her.

Why were charges originally brought? Why did she spend almost two-and-a-half years in prison?

Ms Brady McCarthy’s lawyers claim there was a rush to determine the cause of death. Authorities had a dead baby and needed an explanation. Her lawyers also claim there was a rush to arrest and charge the Irishwoman without proper investigation and without proper examination of the baby.

They say that the presiding doctor at the children’s hospital decided after 24 hours it was shaken baby syndrome and no further examinations were carried out.

She was denied bail upon arrest and continued to be denied on on several occasions until May of this year when she was finally released from custody at the tough Framingham women’s prison in Massachusetts. Authorities had repeatedly claimed they had a “fear of flight” due to Ms Brady McCarthy’s illegal status.

She was only released on bail and placed on house arrest when the charging agency ICE – Immigration and Customs Enforcement – guaranteed Judge Maureen Hogan that she would be deported as soon as the criminal proceedings were concluded.

How did she come to be cleared at this late stage?

Defence lawyers Melinda Thompson and David Meier were belligerent in appealing for a review of all of the medical evidence in the case.

They repeatedly pointed to the conflicting expert evidence as to causes of the baby’s injuries. There was also the baby’s history of medical complaints – she had been sick most of her life and she was treated by her paediatrician at least once a month over the 12 months of her short life for a wide variety of medical issues including stomach bugs, a failure to gain weight – including a diagnosis of being malnourished just five weeks before her death. In addition, there was a suspicion of potential bleeding and bruising disorder.

Furthermore, originally in 2013, prosecutors for the Commonwealth of Massachusetts had said that Ms Brady McCarthy had either slammed or shaken the child to cause spinal injuries, but a year later court documents show there was evidence from the prosecution’s medical experts that someone had inflicted spinal fractures on the baby three-four weeks before her death – at a time when the baby was travelling with her mother in Pakistan and the Ms Brady McCarthy was in Massachusetts.

And so, with sustained requests from the defence lawyers, eventually the Office of the Medical Examiner took the unusual step of carrying out a re-review of the evidence – that was originally slated for just four weeks but took almost four months.

Resulting from that review, the Office of the Medical Examiner changed the cause of death late last week – having originally found blunt force head trauma was the cause of the death, and ruled it as a homicide, she now changed that on the death certificate as an “undetermined” cause of death.

In doing so, she explained that the reason for the change was that she had additional information not available to her prior to reaching the original conclusion – such as additional transcripts of police interviews, grand jury testimony, additional medial records and additional lab tests.

She said she was “no longer convinced” that the bleeding on the brain “could only have been caused by abusive or inflicted head trauma”, could have been related to an accidental injury in a child with a bleeding risk, or could have been pre-existing, undiagnosed disease.

As a result of the change in the cause of death, Middlesex County District Attorney Marian Ryan entered a nolle prosequi in the case, meaning no prosecution would be brought, saying that her office could not “meet the burden of proof” and so could not go forward with the murder charge.

What happens now?

When she was originally released on bail, Judge Maureen Hogan was given cast-iron assurances that Ms Brady McCarthy would be deported as soon as the criminal proceedings were concluded one way or another. ICE has given that guarantee.

My understanding is that Ms Brady McCarthy is hoping to try to make some sort of deal with the authorities that she will face no further charges relating to her illegal status if she surrenders herself and leaves the country immediately.

She has already served almost two-and-a-half years in jail for a crime that not only did she not commit, but that the Medical Examiner has found did not happen in this case.

But of course there are lots of questions for the DA – how the office charged a woman with murder and locked her up for such a long time with evidence that seems to have had such significant holes in it, which that were pointed out repeatedly by the defence lawyers.

Also, during proceedings, allegations were made by the prosecutors against the Irish Government.

Prosecutors said the Government had plans ready to sneak the Co Cavan woman out of the country if she was released from custody and would provide her with a special passport to do so if her passport was confiscated. The allegations were rejected as baseless by the Government.

Has there been any reaction from the baby’s family?

The baby’s parents set up a charitable foundation in her name shortly after her death. They have not made any public comment at this stage. The DA met with them yesterday morning to explain what was happening before making her decision to drop the murder charge public.