No-one will be prosecuted over an outbreak of Legionnaires' disease in Edinburgh which killed four people, the Crown Office has announced.
A total of 92 cases were identified during the outbreak in 2012.
However, an investigation has been unable to identify the source of the bacteria.
Lawyers representing 40 people affected by the outbreak said it was now "crucial" that a fatal accident inquiry (FAI) is held into the case.
The Crown Office and Procurator Fiscal Service (COPFS) said it would consult the families before it made a decision on whether or not to hold an FAI.
Elaine Russell, a partner at Irwin Mitchell Scotland, the firm leading the legal case on behalf of the victims, said: "We have repeatedly called for more information to be shared with the victims but have been met with a wall of silence for years.
"It is embarrassing that they have had to wait so long for the authorities to investigate and share their findings.
"Three years ago four people lost their lives and almost 100 suffered from Legionnaires ' disease, yet the authorities are no closer to knowing what the source of the illness was."
The company's clients include Catherine McDonald, the partner of Bert Air, one of those who died in the outbreak.
She said she was "hurt, angry and disappointed" at the outcome of a meeting between the families and the Crown Office in Wednesday.
"I simply cannot express the frustration that I feel," said Ms McDonald.
"We have waited so long to reach this point but I don't feel we have been provided with any answers as to what happened. I still want to know why Bert died."
Patrick McGuire, a partner with Thompsons Solicitors who is representing nine families affected by the outbreak, said: "This is very disappointing news from the Crown Office.
"This mass poisoning took place in our capital city and yet no one has been brought to book.
"My legal team will now begin immediate work on civil legal proceedings but the Crown Office must also convene a fatal accident inquiry into the outbreak to provide answers for the victims and to stop this ever happening again."
The Health and Safety Executive (HSE) said the investigation, which saw a team analyse samples from several sites, was one of the most complex it has ever undertaken.
The probe saw a number of companies reported for health and safety breaches unconnected to the outbreak.
It is understood a cluster of cooling towers in the south-west of the city formed part of the inquiry.
Legionella bacteria are commonly found in sources of water such as rivers and lakes.
They can end up in artificial water supplies such as air conditioning systems, water services and cooling towers.
Legionnaires' disease is contracted by breathing in small droplets of contaminated water. It is not contagious and cannot be spread directly from person to person.
The investigation into the outbreak involved the HSE and the police.
Gary Aitken, head of the health and safety division at the COPFS, said: "Following a complex and thorough investigation which involved detailed genetic analysis we can only conclude that there is no scientific basis for any prosecution related to the deaths and as a result no criminal proceedings are instructed by crown counsel.
"This was always going to be a difficult and complex investigation due to the number of potential sources in the Gorgie area but we continued on in the hope that the necessary scientific evidence would come to light. Unfortunately that hasn't happened.
"We will now consult further with the families before making any decision in relation to a fatal accident inquiry."
Alistair McNab, HSE head of operations in Scotland, said: "This was the largest outbreak in Scotland in the last 10 years and one of the most complex HSE has investigated, involving visits to multiple sites and dutyholders including contractors and sub-contractors to check compliance with Legionella control standards.
"As HSE and public health experts made clear at the time of the outbreak the source may never be conclusively identified, based on our experience from previous outbreaks.
"This can be due to the fact that Legionnaires' disease can have a long incubation period of up to 19 days, so by the time an outbreak is notified to HSE and other regulatory bodies and sampling carried out on water systems, the bacteria levels may have changed or the source producing bacteria may have ceased operation.
"In addition, as a precautionary measure to prevent further ill health when an outbreak is declared, companies are encouraged to shock-dose their cooling towers with chemicals, which again can prevent positively identifying the source."