Dr. John Carlo with the Dallas County Health Department says they now know what sickened 18 people at Lake Highlands High School on Tuesday: The teachers and staff members have tested positive for marijuana. He says the tests on the muffins showed the presence of THC, a substance that can be extracted from the Cannabis plant. Carlo also says he expects the muffins will be further tested to see what other chemicals might be in them.

Meantime, the FBI is asking for the public's help in identifying the man who apparently delivered the tainted muffins at 8:15 a.m. Tuesday. Anyone with information about the case or the identity of the delivery person is asked to call Dallas FBI at 972-559-5000.

The investigation is a joint effort of the Dallas Police Department, Dallas County Health and Human Services, the Food and Drug Administration, and the Dallas FBI's North Texas Joint Terrorism Task Force.