NEW HAVEN, Conn. - Yale University officials say a laboratory chemical has been detected in a coffee machine suspected to have caused four people at the university's medical school to fall ill.

University spokeswoman Karen Peart said Tuesday independent lab tests on a single-service pod-style coffee machine at the medical school indicated the presence of sodium azide.

She says the substance is commonly used in labs as a preservative and symptoms of exposure to it include headaches, nausea, vomiting and rapid breathing and heart rate.

The university says the coffee machine at the focus of the Feb. 28 incident wasn't connected to a water source and the surrounding area has been declared safe. But it's unclear how the substance ended up in the machine.

University police are investigating.