CREVE COEUR — The agribusiness giant Bayer, concerned a local employee may be infected with the coronavirus, said Monday that it was temporarily closing its Creve Coeur campus for precautionary cleaning, along with three other sites — one in downtown St. Louis and two complexes in New Jersey.

The company learned on Sunday that an employee in Creve Coeur was “considered a person under investigation for coronavirus infection,” and sent an email to employees that night. The case remained unconfirmed as of Monday afternoon, the company said.

If confirmed, it would be the second in the state. A woman in her 20s who had studied abroad in Italy tested positive for the coronavirus after returning home to the area, St. Louis County officials announced Saturday.

Bayer was one of many area businesses and institutions that ramped up precautions this week in the wake of the local case. Several canceled nonessential travel. Some increased facility cleaning or the strength of their disinfectants. At least one other company identified an employee who might have been exposed, and sent him home. Others asked employees to self-report potential exposure.

Experts warned that coronavirus symptoms are often mild. And it’s still unclear how easily it’s transmitted.

“We don’t understand the full spectrum of the disease yet,” said Enbal Shacham, a professor at St. Louis University’s College for Public Health and Social Justice. “We don’t know how transmissible it is.”