As seriously ill patients began to trickle into local emergency rooms yesterday, authorities at New Jersey faced the grim reality that a deadly biological attack had taken place in the tri-state area.

Luckily, it was only a test.

In a five-day drill that began yesterday, thousands of officials and emergency service workers tested their response to a simulated chemical and biological terrorist attack at New Jersey, Connecticut, Canada, and the United Kingdom.

It was the third such simulation to take place under a congressionally mandated series of terrorism-preparedness exercises, the Topoff Project, and homeland-security officials have already begun to call the episode the “most comprehensive terrorism-response exercise conducted in the United States.” Despite New York’s absence from the exercise, the maneuvers could give a boost to the city’s disaster preparations.

“New York is always in some stage of the practice, planning, or critiquing process,” a spokesman for the city’s Office of Emergency Management, Jarrod Bernstein, said. “Topoff is a really good way to hit jurisdictions that don’t necessarily have large-scale drills of their own, but which would still be impacted by an attack.”

The fictional scenario is plausible.

A biological attack, bound for New York City, is uncovered by New Jersey homeland-security intelligence. Feeling the heat of the investigation, the terrorist cell steps up its schedule, releasing a pneumonia-like plague into the air over a Union, N.J., college campus from the safety of a passing sport utility vehicle.

Later in the afternoon, a chemical explosion leaves a section of Connecticut’s New London waterfront, a vital deepwater port, strewn with twisted metal, overturned buses, and hundreds of injured people.

According to estimates from the Department of Homeland Security, the exercise will involve approximately 10,000 participants from more than 275 government and private organizations and 27 federal agencies, including first responders, state police, hospitals, doctors, hazardous-materials teams, and even the news media.

The estimated cost: $16 million, to be billed to the federal government.

While the drill may be new territory for some of the smaller municipalities involved in the project, the simulated maneuvers will help support drills that have become routine in New York since after September 11, 2001, officials said.

“New York City and New York State have put a tremendous amount of effort into terrorism readiness,” a spokeswoman for Homeland Security’s Topoff program, Valerie Smith, said. “Providing different types of scenarios can test all sides of a response.”

In the past year alone, the city’s Office of Emergency Management has conducted three “table top” planning sessions, in which city leaders met to create planned responses to various disaster scenarios. The office also has staged two highly publicized simulations.

Most recently, in May, city officials staged the emergency response to a simulated large-scale explosion at the Bowling Green subway station.