PITTSBURGH (KDKA) — Four more Port Authority employees have tested positive for coronavirus.

Three of the employees tested positive on July 4 and one tested positive on Wednesday. In a release, the Port Authority says the positive cases bring the total number of employees to 21.

One of the employees is a mechanic at South Hills Junction, another is a bus driver who works out of the East Liberty garage, the third is a driver out of the West Mifflin garage and the fourth is an auto shop employee at South Hills Junction.

“As soon as these employees reported that they were self-quarantining pending the results of their tests, management at their respective work locations ordered a deep cleaning and sanitization of the facilities, and where appropriate, the last vehicles they drove. Port Authority also contracted a third-party company to deep clean and sanitize these areas,” the Port Authority said in a release. “Contact tracing investigations are underway for each of the employees, and coworkers with whom they were in close contact will be held off work as a precaution until they are medically cleared to return.”

The Port Authority says it will also begin mandatory on-site temperature screenings at the three work locations.