PUBLIC health chiefs are desperately trying to establish the source of a deadly outbreak of Legionnaires' disease in Scotland's capital that has killed a man and left 15 other victims critically ill.

Dozens more cases are expected over the coming days with 32 suspected or confirmed cases now under investigation by NHS Lothian in Edinburgh.

A man aged in his 50s, who had underlying health conditions, died at the city's Royal Infirmary yesterday.

A total of 13 men and two women aged between 33 and 74 are critically ill in local hospitals, although one man has recovered and been discharged. The health board is now investigating 17 confirmed cases and 15 suspected cases – 10 men and five women.

Last night, Professor Hugh Pennington, the eminent bacteriologist, said more cases could emerge beyond the middle of June.

The health board is concentrating its search for the source of the outbreak on a four to seven-mile radius in the Dalry, Gorgie and Saughton areas of Edinburgh.

Health chiefs have treated air conditioning cooling towers at four sites, although other possible sources are not being ruled out.

It could take up to 10 days before tests on samples taken from the cooling towers are able to determine whether or not they are responsible.

Legionnaires' disease has an incubation period of up to two weeks.

Prof Pennington, emeritus professor of bacteriology at Aberdeen University, said: "There could be more cases appearing even if the source is gone.

"It may be the source is not one of the cooling towers and so the source may still be there, which would mean more new cases."

Health Secretary Nicola Sturgeon will be updated on the situation when she chairs a specially convened meeting of the Scottish Government's Resilience Committee this morning.

A spokesman said: "Our thoughts are with patients and their families at this time and, in particular, with the family of the patient who has very sadly died.

"NHS Lothian and Health Protection Scotland are taking all appropriate steps to manage this situation and are working closely with Edinburgh City Council and the Health and Safety Executive to identify the source as quickly as possible."

The majority of the confirmed cases are linked geographically, and investigations into the other cases, and possible links with the area, are ongoing.

In a statement, NHS Lothian said: "Industrial cooling towers have been identified as a potential source of the infection and the cooling systems at four facilities in the area have been subject to an additional chemical treatment.

"Further inspections of these facilities will be carried out by the Environmental Health Service and the Health and Safety Executive over the coming days to ensure control measures continue to be effective."

Dr Duncan McCormick, consultant in public health medicine and chairman of the Incident Management Team, said: "I would like to express my sincere condolences to the family of the patient who died.

"Investigations into the possible source of this outbreak are ongoing. Meanwhile, medical staff have been actively identifying possible cases to allow us to ascertain the full extent of this outbreak.

"I would like to reassure the public that household water supplies are safe and that Legionnaires' disease can't be contracted by drinking water. Older people, particularly men, heavy smokers and those with other health conditions are at greater risk of contracting the disease. I would urge anyone who develops symptoms of Legionnaires' disease to contact NHS 24 or their GP."

The number of cases has spiralled since the first was identified last Thursday. The symptoms can begin anytime from two weeks after exposure to the bacteria.

Scotlands's worst outbreak to date occurred over the festive season in 1987, when more than 250 children and adults in Lochgoilhead, Argyll, fell ill between Christmas and New Year. In that case, there were no deaths.

The outbreak was linked to the village sports centre. Initially dubbed "Lochgoilhead flu", it was later identified as being caused by the Legionella bacteria.

In April 2008, more than 200 holidaymakers who stayed at the Piperdam Golf and Leisure Resort near Dundee were alerted to a risk of Legionella infection after a male guest died from the disease after returning home. The bacteria was subsequently traced to a shower head and a hot tub at the complex.

Scotland's last major outbreak saw three deaths and nine people infected with Legionnaires' disease in the NHS Lanarkshire and Glasgow areas, although the source was never identified.