Even so, the series of events raised questions about just how far the country has come in its quest to construct a system to identify cases of fast-spreading infectious disease coming through its borders.

The debate began last year, when a Liberian man infected with Ebola, Thomas Eric Duncan, walked into a Texas hospital and was mistakenly released, even though he had said he had recently come from West Africa. He eventually died, and in the process infected two nurses who had cared for him, prompting a national reckoning and challenging the assumption that American hospitals were prepared to treat such complex diseases that had little precedent in the United States. The nurses eventually recovered.

The case in New Jersey was smoother in a few important ways. Health officials identified the man at Kennedy International Airport, where he arrived on May 17 from Liberia, via Morocco. He did not, at the time, have a fever. Federal authorities then handed off his case to local health authorities in New Jersey to begin monitoring in accordance with protocols created last fall, according to the Centers for Disease Control and Prevention. Liberia was declared Ebola-free on May 9, but the travel protocols that were set up last year remain in place.

The man, who was 55, lived in Essex County and frequently traveled to Liberia on mining-related business, health officials said. He showed up at a hospital on May 18, complaining of a sore throat, fever and tiredness. He was asked about his travel history but did not tell the hospital that he had been to West Africa recently, the C.D.C. said. He was released the same day.

In New Jersey, the crossed wires seem to have been between the local health department responsible for the monitoring and the hospital that originally treated him. One looming question is whether the hospital knew that local health authorities had already been assigned to monitor the man; conversely, it was also unclear whether the local health authorities knew the man had gone to a hospital.