The NFL worked Sunday to assess a series of positive COVID-19 results that have all been traced to the same laboratory in New Jersey, raising concerns about the efficacy of the testing program established to minimize the spread of the disease.

“Saturday’s daily COVID testing returned several positive tests from each of the clubs serviced by the same laboratory in New Jersey,” the NFL said in a statement Sunday morning. “We are working with our testing partner, BioReference, to investigate these results, while the clubs work to confirm or rule out the positive tests.”

All told, the NFL is investigating 77 test results from a total of 11 teams, a source told ESPN. The league is rerunning each test with the original sample and hasn’t found any true positive cases. All 77 individuals also took point of care tests, all of which returned negative. NFL chief medical officer Allen Sills was slated to have a call Sunday night with team medical officials to share information and answer questions.

The Chicago Bears announced that they received nine positive results from their Saturday test regimen. All nine were determined to be false positive results, meaning none was an actual indication of infection. The Bears moved their morning practice to Sunday afternoon “out of an abundance of caution” while they sorted through the test results.

The Cleveland Browns said they received results that indicated multiple “presumptive positive” tests from players, coaches and support staff. However, upon retesting, the team found those to be false positives, “which is consistent with the irregularities across the league from multiple teams.” After earlier canceling practice and closing their facility, the Browns said they reopened the building to “resume football activities” Sunday.

The Buffalo Bills pushed the start of their practice back by one hour, with general manager Brandon Beane confirming that some players were to miss Sunday’s practice. Sources told ESPN’s Adam Schefter that quarterback Josh Allen was among the NFL players to miss after receiving a false positive test, and he is expected to return Monday.