The NFL is investigating several positive coronavirus tests from clubs serviced by the same laboratory.
With the 2020 season set to get under way on September 10, the NFL was dealt a setback following a number of positive results from Saturday's round of testing.
However, with all of those coming from one lab that serves a number of franchises, the league announced it was looking into the matter as teams initiated precautionary measures.
An NFL statement read: "Saturday's daily COVID testing returned several positives tests from each of the clubs serviced by the same laboratory in New Jersey.
"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 other laboratories used for NFL testing have not had similar results."
The Cleveland Browns announced they had been informed of positive results among players, coaches and support staff, though none were displaying symptoms.
Despite concerns over the accuracy of the tests, Cleveland have ordered those who were shown to have contracted COVID-19 to self-isolate.
A Browns spokesperson said: "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.
"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.
"As per plan procedure, 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.
"Our facility is currently undergoing a thorough deep-cleaning and disinfecting process and is closed to all personnel for the day. We will provide updates as testing results are confirmed and more information is available."
The Chicago Bears were informed of nine positive results from Saturday's testing but said on Sunday they had been confirmed as false positives.
Buffalo Bills general manager Brandon Beane revealed they were one of "10 or 11" teams dealing with the issue and would consequently be missing players during practice on Sunday.