NEW YORK (AP) — Health officials say two more American aid workers arrived in the United States on Wednesday night to be monitored for Ebola.

The two bring to 17 the number of Americans flown back from West Africa's Sierra Leone since Friday for monitoring. None have tested positive.

All are connected to an unidentified American who returned to the U.S. last week after he came down with Ebola. He is in critical condition at a government hospital in Bethesda, Maryland.

The other aid workers are staying near hospitals with special isolation units in Bethesda, Atlanta, and Omaha, Nebraska, in case they get sick.

However, a Centers for Disease Control and Prevention spokeswoman on Wednesday said two are now considered to be at low risk. They will go home for the 21-day monitoring period.