A catalogue of "basic failings" led to a Legionnaires' disease outbreak that killed seven people, a report says.
Another 180 people were infected in Barrow, Cumbria, in 2002 because of a faulty air conditioning system at the council-run Forum 28 arts centre.
Poor communication at Barrow Borough Council was to blame, said the Health and Safety Executive (HSE).
The council said it was unable to comment due to an internal disciplinary process and ongoing litigation.
An independent inquiry into the outbreak, carried out in 2006, found safety documents had been thrown away.
On Tuesday the inquiry's independent chairman Colin Pickthall told the BBC: "The guidelines on how to avoid legionella are readily available then, as now. They weren't used, they weren't disseminated around the council.
"It was quite obvious that communication at the council was non-existent and the chain of command and duty of care completely failed"
"They were put in bins or they were put in files."
The errors could have been "easily prevented", said the HSE's report, which drew upon the inquiry's findings.
Barrow Council was later cleared of corporate manslaughter following the deaths, but fined £125,000 for breaching health and safety laws.
Andy Macauley, whose grandfather Richard died in the outbreak, said: "It was quite obvious that communication at the council was non-existent and the chain of command and duty of care completely failed.
"The £125,000 fine will have been nothing more than an inconvenience to the council and will have had little impact on their budget."
During July and August 2002, visitors to Forum 28 were exposed to Legionella bacterium spilling out from the air conditioning unit.
The bacteria had been able to breed because the system was not properly cleaned and the temperature inside was not controlled.
'Fateful coincidences'
The HSE report comes after a lengthy investigation and a series of public meetings in Barrow last December.
It concludes: "The number of fateful 'coincidences' involved was scarcely credible.
"Such basic failings should not have occurred and, sadly, these failures could have been easily prevented."
Investigators said communication between staff responsible for the air conditioning system at the borough council was poor and that they failed to act when concerns were raised.
They also failed to carry out a risk assessment and to properly manage contractors to ensure the system was adequately maintained, the report adds.
Families' anger
The report also highlights the role of the council's design services manager, Gillian Beckingham.
Last year she was convicted of health and safety breaches at Preston Crown Court, but cleared of manslaughter.
She had, the court heard, not properly maintained the contracts that provided for the upkeep of the air conditioning unit.
The HSE said Ms Beckingham's failures were "more significant than others".
Richard Macauley, 89, Wendy Milburn, 56, Georgina Sommerville, 54, Harriet Low, 74, Elizabeth Dixon, 80, June Miles, 56, and Christina Merewood, 55, all died in the outbreak.
Their families have repeatedly called for council chief executive Tom Campbell to resign.
An inquest into the deaths is to take place in June.
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