The Philadelphia Department of Public Health is investigating two more cases of E. coli that were reported Friday.

That brings the total number of people sickened in the outbreak announced last week to 16.

The city Health Department, however, is not yet prepared to identify which restaurants are involved in the outbreak.

“The Health Department’s investigation, like all food-borne illness investigations, takes time,” said James Garrow, spokesperson for the Health Department.

“This a wide-ranging investigation that asks very sick people to list every single thing that they’ve eaten or drank, at the level of every individual ingredient, during the period when they were exposed to the bug,” Garrow said. “This generates a massive list of possible items and locations that may have been the culprit.”

The individuals’ lists are then compared to try find a common food or drink item, and sometimes one is never identified, he said.

“In those cases, people just stop getting sick and then the investigation is over,” he said.

The Health Department said last week the illnesses were due to Shiga-toxin E. coli, one of five E. coli strains. Symptoms usually start with non-bloody diarrhea, which can progress to bloody diarrhea after two to three days. Severe abdominal pain and fever may also occur.

Exposure to the bacteria often occurs through contact with food or water contaminated by human or animal stool, or through contact with an infected person. Outbreaks have been associated with consuming undercooked beef, unpasteurized milk, or raw leafy vegetables.