A half-dozen NFL clubs experienced COVID-19 test result irregularities from the same New Jersey laboratory, prompting teams to change or cancel practice sessions on Sunday as a precaution.
The league announced several positive results for multiple clubs were returned from the same lab Saturday, causing questions given a positivity rate below 1% from more than 100,000 coronavirus tests conducted since the start of NFL training camps.
"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.
"We are working with our testing partner, BioReference, to investigate these results, while the clubs work to confirm or rule out the positive tests.
"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."
The league said other labs used to process NFL test results have not returned similar results.
The Browns closed their headquarters and called off practice so COVID-19 cleaning and precautions could be undertaken.
"After consistently receiving encouraging results from daily testing since returning to the facility in July, the Cleveland Browns' COVID-19 testing process conducted yesterday initially indicates multiple individual presumptive positive cases that include players, coaches and support staff," the team said in a statement.
"Although, we have received indication from the NFL that they are investigating irregularities in lab results that have also impacted other clubs, out of an abundance of caution, we will immediately follow our comprehensive Infectious Disease Emergency Response plan and pause any activity in our building today and conduct meetings virtually.
"The team is also diligently reviewing proximity data for contact tracing and instructed self-isolation for those who tested positive and others as appropriate even though they all remain asymptomatic."
The Chicago Bears postponed a morning workout to the afternoon after nine individuals, including players and staff members, tested positive.
"We followed additional NFL-NFLPA testing protocol and confirmed all nine results as false positives," the Bears said in a statement. "Out of an abundance of caution, we postponed this morning's practice to this afternoon."
The New York Jets had 10 false positives and canceled a walk-though Saturday before learning Sunday morning that a practice could be held.
Steelers general manager Kevin Colbert said six players missed practice but none were placed on the COVID-19 reserve list.
Bills general manager Brandon Beane said "key players" would miss Sunday workouts because of irregularities in testing while Vikings coach Mike Zimmer said eight players, three staff members and one coach had "presumptive positive" tests and would miss Sunday's practice.