Portuguese authorities said on Tuesday cooling towers at a fertilizer plant were the likely source of an outbreak of Legionnaires' disease that has killed five people and infected over 200, making it one of the world's largest ever outbreaks.

Environment Minister Jorge Moreira da Silva told reporters all cooling towers in the affected area near Lisbon had been shut since Sunday to bring the flare-up under control.

Health Minister Paulo Macedo said the detection of new cases was decelerating, but there will be more in the coming days.

Health ministry data released late on Monday showed the number of those infected had jumped to 233 from 160 the previous day, and one more person had died. Three dozen were in intensive care in various hospitals.

The authorities were investigating the "possible environmental crime of releasing microorganisms" into the air by Adubos de Portugal, a unit of Spanish company Fertiberia, in the affected area of Vila Franca de Xira, according to Moreira da Silva.

Company officials were not available for comment.

He said samples collected in the last few days allowed the authorities to "consider with a large degree of certainty that (the bacteria) are associated with cooling towers, not with the water network, and in particular with one installation".

Four parishes in the district of Vila Franca de Xira, about 25 km (16 miles) northeast of Lisbon, had the greatest concentration of the cases. One of the infected was a man in Porto, 200 miles to the north, who had worked on the fertilizer plant's cooling tower.

The disease is contracted by breathing in a mist or vapor contaminated with the Legionella bacteria, which can grow in cooling towers, showers, hot tubs and other water sources. It is not transmitted directly from person to person.

People already in poor health are the most vulnerable.

While the flare-up is far from being among the world's deadliest, it is already the third-largest by confirmed cases after the 2001 outbreak in neighboring Spain, with some 450 cases and six dead, and the 1999 outbreak in the Netherlands, when 32 people died and more than 300 were infected after visiting a flower exhibition in the town of Bovenkarspel.

The illness is named after a 1976 outbreak at an American Legion convention in Philadelphia, which killed 34.