Brian Taylor, 75, from Huddersfield, died last month – just weeks after holidaying at the Hotel Kalofer.
John Cowan, 43, from Lanarkshire, was also staying at the same hotel in June and has also died from Legionnaires’ disease.
Holiday firm Jet2 said two sets of independent tests had found no evidence of contamination at the hotel.
Both families are threatening civil legal action against the flight and holiday operator.
Mr Cowan died on 19 June, a week after returning home, as a result of Legionella bacteria he contracted while on holiday with a friend.
His heartbroken family made the decision to switch off his life support machine at the Golden Jubilee National Hospital in Clydebank, West Dumbartonshire, after John suffered catastrophic brain damage.
Mr Cowan, who was a hotelier, had been holidaying Bulgaria from 4 to 11 June, staying in Hotel Kalofer in Sunny Beach resort.
Four others, including Mr Taylor, are believed to have contracted the same virus.
Mr Taylor’s stepson Martin Farrell told the BBC: “We are absolutely devastated and heartbroken. No-one expects to go on holiday and catch this disease.”
Mr Farrell said he felt like Jet2 had “washed their hands of the situation” and demanded answers from the holiday firm.
“It feels like they want to sweep it under the carpet,” he said.
“We want Jet2 to come forward and tell us why it happened, how it happened and what steps have been put in place to prevent it from happening again.”
A spokesperson for Jet2 said: “We would like to offer our heartfelt condolences to their families at this very difficult time and we have been in constant contact to offer all the assistance and support that we can.
It added: “We will continue to assist the local authorities in their investigations as required.”