Independent investigators are to carry out extensive testing at the Berkeley apartment complex in California this month, where a balcony collapsed in June, killing six young Irish people and injuring seven others.

A criminal investigation led by the District Attorney's office has been under way since shortly after the tragedy.

It is said to be "progressing well" and is about to move into a phase of "major testing" of the balcony and the building.

Those conducting the criminal investigation are said to be have been surprised and shocked at how the wooden joists holding the balcony deck in place had been "so completely waterlogged".

A team of five lawyers and five investigators from the District Attorney's office is actively working the case.

In addition, outside engineering, architecture and water-proofing experts have been hired by the DA's office to carry out an independent professional examination of the building, the collapsed balcony, and the balcony beneath it removed following the tragedy for safety reasons.

The experts carried out a preliminary examination of the scene on 29 July, and will return within the next month to carry out major testing, which will involve erecting scaffolding and is likely to last several days

In addition there will be forensic testing of both balconies, something the initial City of Berkeley investigation did not do.

The Chief Assistant District Attorney Kevin Dunleavy said the team was trying to carry out a thorough investigation while being mindful that the families of those killed and those who were injured would like an outcome as soon as possible.