It is unlikely that a prisoner died from Legionnaires' disease, the governor of Magilligan Prison has said.
The bacteria was found in the jail's healthcare centre
The terminally ill inmate was moved to Causeway Hospital on 29 January and tested positive for Legionella Bacterium. He died last week.
Water at the HMP Magilligan's health centre was found to contain the bacterium.
Prison governor Tom Woods said he does not think the death was connected to this.
"The gentleman concerned had serious medical complications and was terminally ill," he said.
"We haven't seen the post mortem results, he would have been with a low immune system very susceptible to different viruses and bugs.
"Until we get the definitive post mortem results we'll not really know, but at this stage the indications are that he did not die of Legionnaires' disease."
Six prisoners have been moved. It is understood that none have flu-like symptoms associated with the disease.
Legionnaires' disease is a type of pneumonia or lung infection. The major source of infection is water distribution systems in large buildings.
In a statement, the Prison Service said the men were moved after consultation with the Health and Safety Executive for Northern Ireland and the Western Health and Social Services Board.
"Discussions are ongoing to identify what further action may need to be taken," the statement said.
There are 420 sentenced prisoners at the County Londonderry jail, which was first opened in 1972 on the site of an army camp.
Initially it housed paramilitary prisoners, until they were moved to the Maze in 1977 when Magilligan became a 'normal' prison.
Magilligan has its own 8-bed health care centre but prisoners can be referred to outside hospitals for treatment by the medical staff.
The jail is due to be replaced with a new £150m prison with up to 1,000 individual cells to house high risk offenders.