Police face major hurdles before being able to make any kind of prosecution over the collapse of the CTV building, a lawyer for families of victims says.
The Christchurch building collapsed in the February 2011 earthquake, killing 115 people. Until yesterday, police had described their work on the case as an assessment but have now upgraded it to a criminal inquiry.
Police are not likely to decide until next year whether to lay charges.
Nigel Hampton QC, who represented at an inquest families of those who died, told Radio New Zealand's Morning Report programme police had to show proof of gross negligence by a particular person, then causation.
"They have to show - prove to the criminal standard - that that alleged gross negligence was the cause of the collapse and therefore the deaths of the people inside that building."
Murry Grant, who lost his wife in the building collapse, was surprised the police were taking things further.
"A lot of us are hoping that somebody is going to be held accountable, " Mr Grant said.
"Whatever comes out of that comes out of it, and then I think a lot of people will move on ... at the moment I feel like it's really not finished."
More than 18 months of assessments and expert advice from engineering firm Beca led police to investigate further.
A team of nearly 20 people are working on the inquiry full time, including eight investigators.
Two designers of the CTV building along with others who gave evidence to the Canterbury Earthquakes Royal Commission will be interviewed by police as part of the inquiry.
Detective Superintendent Peter Read said the inquiry could could lead to a charge of manslaughter. "Bearing in mind that 115 died, that's the principal charge that we're concentrating on. It's got to be supported by facts and we're still a long way from that yet."
Information from the Royal Commission will be used but the next step is to fill in the evidential gaps and make an assessment against the law. "I don't expect that'll be done this year, I'm looking into early next year," Detective Superintendent Read said.
The Royal Commission in 2012 found that the CTV building had serious design and construction flaws, and its report and heavily criticised designers Alan Reay and David Harding.
Both escaped sanction from the Institute of Professional Engineers after resigning from the organisation.
Structural Engineering Society executive committee member John Hare said those in the profession are being judged by a situation involving a small group of people.
"I think from any perspective what happened back on February the 22nd was absolutely a story of human tragedy for a lot of people, and most of the profession would tell you they would have done anything they could have to have not had that."
Mr Hare said things had changed since the CTV building was designed and given a permit.