Thousands of supporters of President Donald Trump showed up in an indoor arena on Saturday night for a rally that some fear could help fuel nascent spikes in COVID-19 cases, concerns that were amplified after six staffers helping to set up the event tested positive for the virus.

Officials of the state and city health departments were already bracing for a possible surge in cases from large outdoor demonstrations against police brutality held across the country. Now the Trump rally in Tulsa, Oklahoma, shaped up to be the first indoor event of such a massive scale since the pandemic took hold.