The Alameda County District Attorney has promised the families of those killed and injured in the Berkeley balcony collapse that the case will be "thoroughly and exhaustively" investigated.
Nancy E O'Malley said her office has a team of investigators, prosecutors and victim/witness advocates working on the case.
Five Irish students and one Irish-American died when the balcony gave way last week.
Seven other students were injured in the incident.
Speaking at a press conference at her office, Ms O'Malley said the tragedy had devastated the Greater Bay Area and Ireland.
She said the DA's office would be the lead agency in this criminal investigation.
Ms O'Malley said she would be investigating whether there was cause to bring charges of criminal negligence in relation to the balcony collapse and whether there was enough evidence to prove those charges beyond reasonable doubt in a court of law.
She said she had 66 investigators at her disposal and she would also bring in outside experts.
She said her office would also be working with agencies from the State of California.
She said the City of Berkeley had a limited scope in terms of the investigation it had carried out but she could guarantee the families of those killed and those who were injured that she would carry out a complete investigation.
Ms O'Malley said she was happy that the city authority and the police department would be supporting her investigation.
In terms of evidence she said that the collapsed balcony was in police possession and the balcony from the floor beneath was in a secure location guarded by sheriffs.
She said there was no timeline on the investigation but any charges of criminal negligence needed to be brought within three years of the crime.
Despite having declared their examinations to be finished, the City of Berkeley local authority and the police department are now co-operating with the District Attorney's formal investigation.
The City of Berkeley local authority announced on Tuesday that its investigation into the tragic balcony collapse at the Library Gardens apartment complex had concluded.
Its inspectors, it said, were satisfied that extensive dry rot was the sole cause of the failure of the wooden joists holding the balcony in place.
Meanwhile, the funeral of Lorcán Miller took place today while the funerals of Niccolai Schuster and Olivia Burke took place yesterday and Eoghan Culligan and Eimear Walsh were laid to rest on Tuesday in Dublin.
The funeral mass for Ashley Donohoe, a cousin of Olivia Burke, was held in California last weekend.
Two of the seven students injured in the balcony collapse have been discharged from hospital. Two of the injured remain in a critical condition in hospital in California.
