The air traffic control tower at Chicago’s Midway airport was temporarily closed Tuesday after several technicians tested positive for the novel coronavirus, the Federal Aviation Administration said.

The agency said operations would continue at reduced levels. A ground stop — an order halting flights from taking off from or departing for the airport — was issued just after 5 p.m. Tuesday but was lifted soon after.

“The air traffic system is a resilient system with multiple backups in place,” the agency said in a statement. “This shift is a regular execution of a long-standing contingency plan to ensure continued operations. Each facility across the country has a similar plan that has been updated and tested in recent years.”

The FAA said the airport itself remained open. An FAA bulletin said a traffic management program was in effect at Midway and advised travelers to contact their airline for information about delays.

The effects of the virus have been rippling through the aviation system. Several Transportation Security Administration officers across the country have tested positive, and on Monday the FAA disclosed that three other employees had been diagnosed.

The National Air Traffic Controllers Association said the FAA should work with local health authorities and the Centers for Disease Control and Prevention to guarantee that all personnel who work in the Midway tower are immediately tested for the coronavirus.

The association said the control tower should remain closed “until it can be disinfected according to appropriate public health standards,” and union officials have asked to review the cleaning protocols so their own industrial hygienist “can ensure the efficacy of the work.”

Association leaders said that they were “encouraged” the FAA is seeking the “rapid testing” of tower workers and that they “applaud” the agency for moving air traffic control services from the Midway tower to the Chicago Terminal Radar Approach Control facility while the disinfection work is completed.

FAA Administrator Steve Dickson, in a letter to agency staff Monday, described the agency’s first three cases and said he expected more.

“Even with positive cases in our workplaces, our collective level of risk remains in line with the level of risk for all Americans,” he said. “As testing continues across the country, we can expect to see more positive cases among our 58,000 federal and contract employees.”

The first three cases include an aviation safety inspector working in an FAA office in Des Plaines, Ill., that oversees United Express and more than 50 international airlines. After that infection, workers in two FAA offices — located in the same building, near O’Hare International Airport — were instructed to telework.

The FAA said in a statement that it is “working with the appropriate authorities to identify and locate people who might have come into contact with” the inspector and that it has coordinated with airlines about tracing contacts with the employee.

The two other FAA workers known to have tested positive for the virus include an employee of the air traffic control management office at headquarters in Washington, and another who works for the finance and management office in a “full-time telework capacity” in the agency’s eastern region, according to the FAA.

On Friday, Professional Aviation Safety Specialists, a union representing thousands of FAA employees in jobs other than air traffic control, issued a statement faulting the agency for, it said, sharing too little about its continuity plans and taking insufficient steps to enable staff to work remotely. The union said it had written to Dickson twice since March 2 asking for a joint review of the agency’s pandemic contingency plans.

“If the agency won’t share contingency plans with the union and won’t be more proactive and transparent about what it is doing to protect employees, it is more difficult for PASS to assist with a successful recovery from this worldwide emergency,” said Mike Perrone, the union’s president.

Liz Doherty, a spokeswoman for the union, said a conference call between union leaders and the agency Friday did not resolve the situation.

The FAA has faced major disruptions to the air traffic control system before. In 2014, an agency contractor started a fire at a facility in Illinois, causing delays across the country. Air traffic control duties were ultimately handed over to nearby facilities until the center could be brought back online.