Unsanitary conditions at a packing plant that handled fresh cantaloupes from a Colorado farm likely contributed to one of the deadliest listeria outbreaks in U.S. history, health regulators said on Wednesday.

The most lethal outbreak of the foodborne bacteria in more than two decades has killed at least 25 people and sickened 123 others, also causing one woman to have a miscarriage, regulators said.

The U.S. Food and Drug Administration said the design of the packing plant allowed water to collect in pools and made it difficult to clean and sanitize the facility.

The firm was packing cantaloupes under unsanitary conditions where they may have become adulterated, said Sherri McGarry, a senior adviser at the FDA's Office of Foods.

The packing plant is part of Jensen Farms, the source of the tainted melons that were first investigated in late August and have infected people in 26 states.

McGarry said it was impossible to pinpoint exactly how the bacteria got into the plant, which, until the outbreak, had not been inspected by the FDA since it was registered in July of 2010.

Listeria monocytogenes is a frequent cause of U.S. food recalls in processed meats and cheeses, but contamination in fresh produce is a new and worrisome development.

While the number of cases linked to the cantaloupes is now decreasing, health officials said it was too soon to declare the outbreak over, at least until the end of the month. Listeria has a long incubation period, with symptoms sometimes not showing up until two months after people consume listeria-contaminated foods.

The elderly, pregnant women and people with weakened immune systems are most at risk. Symptoms include fever and muscle aches, sometimes preceded by diarrhea and other gastric problems.

The Jensen Farms listeria outbreak is the deadliest since 1985, when poorly pasteurized Mexican-style soft cheese killed 48 people and infected 148 others.

Listeria is present in many parts of the agricultural environment, from decaying vegetation to the fields where food is grown. But it rarely causes serious illness.

The organism needs to get on food and grow to levels where it can cause disease. Since it can grow at low temperatures, that can happen anywhere along the food chain.

The listeria could have come to the Colorado plant from trucks carrying the cantaloupe, or from the fields where the fruit is grown, the FDA said.

FDA officials said the Colorado packing facility was not typical. There's no reason to believe these factors are indicative of practices throughout the industry, McGarry said.

Both the FDA and Jensen Farms said the packing facility would not distribute more food until the agency deemed it safe. The facility is not currently open, since the growing season is over.

Our operation will not resume until we have done everything in our power to ensure the safety of our products, said a woman who answered the phone at Jensen Farms but declined to give her name.