Colleges and universities that reopened for face-to-face instruction might have caused tens of thousands of additional cases of Covid-19 in recent weeks, according to a new study conducted by researchers at the University of North Carolina at Greensboro, Indiana University, the University of Washington and Davidson College.

The researchers estimated that an extra 3,200 cases a day occurred in the U.S. that likely wouldn’t have happened had schools kept classes online…

To determine whether college reopenings influenced case counts in the larger community, the researchers used cellphone GPS tracking data to look at the movements of people—including an influx of students—and calculated the infection rates of the surrounding county from mid-July to mid-September, before and after students showed up.

They found little uptick in case counts for those communities where students moved back to be near campus, but were taking classes online. The biggest surge came near schools with in-person instruction, with particular spikes in places where students came from hot-spot zones elsewhere in the country.