Unsanitary conditions at a Colorado cantaloupe farm's packing facility are a possible contributing cause of one of the nation's worst outbreaks of listeria contamination in food, the Food and Drug Administration said Wednesday.

Meanwhile, a Centers for Disease Control official said it is too early to declare the outbreak over, but the number of new cases appeared to be diminishing.

"The peak in illnesses appears to have occurred from late August until the middle of September," said Dr. Barbara Mahon of the CDC, who added that additional monitoring will be needed for at least another two weeks.

The FDA said it was unable to pinpoint the definitive cause of the outbreak, which has been linked to 25 deaths so far.

Read FDA cites sanitation issues in Listeria outbreak