LAGOS, Nigeria — Nigerian health authorities acknowledged Tuesday that they did not immediately quarantine a sick airline passenger who later died of Ebola, announcing that eight health workers who had direct contact with him were now in isolation with symptoms.

Ebola, which can cause victims to bleed from the eyes and mouths before a grisly death, has killed nearly 900 people in four West African nations.

The outbreak, which emerged in March, spread to Nigeria in late July when Patrick Sawyer, a 40-year-old American of Liberian descent, flew from Liberia’s capital to the megacity of Lagos. The announcement that Sawyer was not immediately quarantined underscores concerns that West Africa is ill-equipped to contain such a disease.

By contrast, two American aid workers who were infected with Ebola in Liberia received an experimental drug and were flown in a chartered jet to Atlanta, where they are being treated in a hospital isolation unit. Ebola concerns in the U.S. have led some worried people to hospital emergency rooms, and prompted Ebola testing of at least six patients, according to the U.S. Centers for Disease Control and Prevention. The tests that have completed have all been negative, the federal agency said Tuesday.

Experts say people infected with Ebola can spread the disease only through their bodily fluids and after they show symptoms. Since the incubation period can last up to three weeks, some of the Nigerians who treated Sawyer are only now showing signs of illness that can mimic many common tropical illnesses – fever, muscle aches and vomiting.

Initially authorities said that the risk of any exposure was minimal because Sawyer was whisked into isolation after arriving at the airport.

But Lagos health commissioner Jide Idris said Tuesday that the nature of his disease “was not known” the first day.