The NFL revealed Sunday that several positive COVID-19 tests were found a day earlier by one of its testing partners.
The New York Jets said they had 10 false positives and the Chicago Bears said they had nine.
The league has asked the New Jersey lab BioReference to investigate the results "while the clubs work to confirm or rule out the positive tests."
The NFL did not identify the teams or say how many tests were positive.
The Cleveland Browns have called off their workout. The NFL uses BioReference for all of its COVID-19 testing, though tests are handled by labs throughout the nation to ensure teams get results quickly.
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(By Associate Press Pro Football Writer Barry Wilner)