It was a dress rehearsal to disaster, giving all the different players a chance to see how they would react to the situation and each other.
A phony alert went out at 8 a.m. that Tank 181 at the ConocoPhillips refinery had collapsed. Clean-up boats immediately took off for the refinery's waterfront perimeter to mop up imaginary oil.
In the middle of the channel, workers on the hulking, 208-foot Delaware Responder skimmed the river's surface by maneuvering a 100-yard boom.
Meanwhile, a few miles away, teams of responders - from FBI agents to local police, state environmental officials, ConocoPhillips managers, emergency workers, and Coast Guard officers - set up a command center at the Embassy Suites near Philadelphia International Airport.
From there, they directed a two-pronged mission: containing the spill and investigating the terrorist strike.
In the script of the exercise, the ConocoPhillips refinery became a giant crime scene, with environmental protection officials from Pennsylvania, Delaware and New Jersey having to figure out how they would work elbow-to-elbow with criminal investigators.
The idea of a terrorist attack on the Delaware River would have seemed remote before Sept. 11. But across the country, the maritime community has been put on high alert, with ports, vessels and riverside assets now viewed as potential targets.
Locally, the terrorist threat is particularly high because of the high concentration of refineries, chemical plants and oil tankers - assets that could be turned into weapons by terrorists.
Refineries in the Philadelphia area process more crude than any other port on the East Coast. The region also has a number of riverfront chemical plants that handle volatile cargo.
"Philadelphia offers a lot of - shall we say - opportunities" for terrorists, said Lt. Cmdr. Nathan Knapp of the Coast Guard's strike team that responds to oil spills and hazardous-material disasters on the Atlantic coast.
"The goal is to train everyone in the port to be prepared," said Knapp, whose unit is based at Fort Dix in New Jersey.
Every two years, the Coast Guard is required to stage oil-spill exercises in ports that handle oil trade. The action was mandated by the Oil Pollution Act of 1990, enacted after the Exxon Valdez accident in Alaska.
After Sept. 11, the Coast Guard began including terrorism subplots, with this year's drill a first for the Delaware River port community.
Security experts say that terrorists in the Middle East have shown a willingness and aptitude for attacking maritime targets. On Oct. 12, 2000, the USS Cole was struck by a small boat loaded with explosives off the coast of Yemen. Two years later, the French supertanker, the Limburg, was rammed, also in Yemen, dumping 90,000 barrels of crude.
Although no one was anticipating such an attack yesterday on the Delaware River, no one was ruling it out either.
"We study it very closely - all the hits that have happened and all that could happen," Knapp said.
Participants said the exercise gave them a chance to react under calm and controlled conditions.
Patrick Prosser, a spokesman for ConocoPhillips, said the effect of the drill would be to "reduce organizational panic" in the event of the real thing.
"Whether or not the terrorist threat is real is not for us to say," Prosser said. But, he added: "It's much easier to react and respond when you know who will be there to help you."
Contact staff writer Jennifer Lin at 215-854-5659 or email@example.com.