Laboratory tests on a pickled beet salad served at a Portland elementary school where more than 20 students later became ill have come back inconclusive, the Maine Center for Disease Control and Prevention said Wednesday.

The tests were part of an investigation into whether the pickled beet salad sickened the 22 students at Portland’s Reiche Elementary School on March 10. The salad was not served at any other city school that day.

School officials said the students became ill about an hour after eating lunch. Sixteen students vomited and six others complained of stomach pain. The school district said in a press release that no student required medical treatment and all but one returned to school the following day.

“The lab test results on the food samples from the Reiche School are inconclusive,” John A. Martins, spokesman for the state agency, said in an email statement. “There has been no determination of the onset of the illness which affected 22 students on March 10. All of the students returned to school last week and remain healthy.”

Martins said the CDC’s investigation into the possible cause of the illnesses is ongoing, but he did not elaborate on that statement or whether the cause of the sickness was likely to be determined.

Meals for all of Portland’s public schools are prepared in the district’s central kitchen on Waldron Way.

The central kitchen passed a health inspection last Wednesday, but the Reiche kitchen failed with four violations that can compromise food safety. School officials have said those violations have been corrected.