A malfunctioning pipe valve at a suburban dairy was the most likely cause of a salmonella outbreak that sickened 18,000 people in six midwestern states, according to a report issued yesterday.

The valve apparently allowed contaminated raw milk to mix with pasteurized milk and had malfunctioned for almost a year at the Hillfarm Dairy in suburban Melrose Park, owned by the Jewel Companies Inc. grocery chain, state Inspector General Jeremy Margolis said.

"This wasn't sabotage. This wasn't a superbug. This wasn't a failure of the pasteurization process," Margolis said at a news conference. "It was a unique microbiological-engineering phenomenon."

The report's findings supported a preliminary report by the task force of state, federal and private experts. It did not accuse the dairy of wrongdoing or negligence.

The salmonella was blamed for two deaths and for contributing to four others.

The Hillfarm Dairy was closed April 9 after the outbreak began in late March.

Jewel's parent company, American Stores Co. of Salt Lake City, has said the outbreak cost it $3.5 million. The costs including closing the dairy, which idled about 140 workers.

Jewel has been accused of wrongdoing in about 300 lawsuits filed since the outbreak.