Investigators say they have done everything but tear out the walls and floors of a suspect dairy, but the cause of a salmonella outbreak that has poisoned almost 5,000 people in five states remained a mystery yesterday.

Health inspectors continued dismantling and testing machinery at the Hillfarm Dairy in suburban Melrose Park, said Susan Weidel, legal counsel for the state Inspector General's office.

Meanwhile, Jewel Companies Inc., which sold the milk from the dairy in many of its 217 stores, has weathered two bomb threats but faces lawsuits and possibly the cost of cleaning up milk that employes improperly dumped into storm sewers.

There have been 4,742 reports of salmonella infection -- which causes fever, nausea and diarrhea -- in Illinois, Michigan, Indiana, Wisconsin and Iowa, Weidel said. At least three deaths have tentatively been linked to the outbreak, she said.

Illinois had 4,408 of the reported cases, with 3,392 of them confirmed.

Jewel spokesman Jim Henson said the company had replaced all products from the Hillfarm dairy with those from a new contractor.

Meanwhile, the Illinois attorney general's office is considering a request from state Environmental Protection Agency Director Richard Carlson to seek damages of more than $100,000 from Jewel because of the dumping of milk into storm sewers. As much as 100,000 gallons of milk may have been dumped into the sewers, rather than into sanitary sewers that would have channeled it to sewage treatment plants, Carlson said.

The salmonella poisonings led to the firing of the Illinois public health director, who Gov. James R. Thompson said had been on a Mexican vacation during the two-week probe of the problem. The Chicago health commissioner also was on vacation during much of the crisis.