The NFL is working Sunday morning 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 spread of the disease.

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

Buffalo Bills general manager Brandon Beane told reporters that at least 10 teams were impacted.

The Chicago Bears announced they received nine positive results from their Saturday test regimen. All nine were determined to be false positive results, meaning none were actual indications of infection. The Bears moved back their morning practice to Sunday afternoon "out of an abundance of caution" while they sorted through them.

The Cleveland Browns, meanwhile, canceled their Sunday practice entirely, saying they received results that indicated multiple "presumptive positive" tests from players, coaches and support staff. The Browns said those who had positive tests remain asymptomatic, but the team said Sunday's meetings would be conducted virtually while the facility undergoes a "thorough deep cleaning and disinfecting process."

The Bills also pushed back the start of their practice by one hour, with Beane confirming some players will be missing from Sunday's practice.

"We're just trying to be very cautious," Beane said. "We've been open with our team. The league jumped on this early to let us know that it wasn't just our team, it's a lot of teams."

The New York Jets had 10 tests come back as false positives, coach Adam Gase said. The team canceled a walk-through practice Saturday night but returned to the practice field Sunday after the re-tests came back negative.

The Pittsburgh Steelers released a statement Sunday saying six players missed practice Saturday due to COVID-19 protocol, but all six will be returning to meetings Sunday and practice Monday. The team did not claim any of the tests to be false positives.

The NFL has been re-testing the tests that came back positive from the New Jersey lab, and so far they are finding evidence that leads them to believe the issue involves testing irregularities, not a surge of positive tests.

In its statement, the NFL said that the affected clubs "are taking immediate precautionary measures as outlined in the NFL-NFLPA's health and safety protocols to include contact tracing, isolation of individuals and temporarily adjusting the schedule, where appropriate."

Any player who tests positive for COVID-19 -- even if it is found to be a false positive upon retest -- must have two negative tests before they will be allowed to return to practice.

NFL officials hope this experience can help inform game day protocols to help ensure that, on game weekends, they can isolate those who are actually sick and leave enough time for confirmatory tests so that they can work around issues like this and games aren't affected by testing irregularities.

"My question immediately would be, if you realize it's a lab issue, would a point-of-care test on the game site -- if it was negative, would that allow the player to play?" the Bills' Beane said Sunday. "And I don't have that answer, but that's the question that we'll have to see how the league would respond to that.

"... Probably better that this happened now than three weeks from now. But it seems like every few weeks, every week, something's going on, and so who knows what the next curveball will be."

The NFL said other BioReference laboratories used for NFL testing have not had similar results to that of the New Jersey lab. NFL testing is conducted exclusively by BioReference, but the tests are serviced by labs around the country to ensure results in 24 hours or less.

The league has avoided team outbreaks during training camp. As of Sunday, only four of the nearly 3,000 players who have spent time in training camp were currently on the COVID-19 list.

ESPN's Dan Graziano contributed to this report.