Washington, D.C., mourning the victims of the Sept. 11 attacks this morning, heard an eerie echo less than an hour later -- reports that a Coast Guard vessel had fired shots at a boat in the Potomac River near the Pentagon.
But after half an hour of anxiety, the Coast Guard and local police said the whole thing was a training exercise, and no shots were fired at all.
The exercise took place before mourners had left the memorial ceremony at the Pentagon where President Obama spoke, and while memorials in New York City and Shanksville, Pa., were still in progress. Mourners in New York were less than halfway through the reading of the names of the 2,752 people who died in the destruction of the World Trade Center.
In the confusion, departures from Washington's Reagan National Airport were halted from 10:08 a.m. to 10:29 a.m., delaying 17 departures, and FBI agents scrambled to the scene near the river. A law enforcement official, asking not to be identified, told the Associated Press the local FBI office had not been told ahead of time about the exercise.
Network news programs broke into regular morning shows to report the incident. CNN, Fox and other outlets went to blanket coverage.
As word spread of the false alarm, Frances Townsend, former Homeland Security adviser in the Bush administration, got angry -- calling the exercise "felony stupidity" in a CNN interview.
Why hold such a drill on Sept. 11? A Coast Guard information officer, John Edwards, said it was part of a "standardization training" drill. He told ABC News the Coast Guard trains this way all the time.
During the exercise, a crew member on a Coast Guard boat said, "Bang, bang, bang," over a two-way radio -- a transmission overheard over a standard Coast Guard hailing frequency by many listeners, including news people, and perhaps misinterpreted as word of actual shots.
A senior Coast Guard officer told ABC News that all transmissions during such exercises are supposed to be preceded by the words, "Exercise, exercise, exercise."
CNN, which had been singled out for jumping on the story, put out a statement Friday afternoon, saying it overheard Coast Guard radio transmissions about a boat that had breached a security zone in the Potomac. It said a Coast Guard spokeswoman, in two calls, could not find anyone with knowledge of "any activity in the area."
"Given the circumstances," said the CNN statement, "it would have been irresponsible not to report on what we were hearing and seeing. As with any breaking news story, information is often fluid and CNN updated the story with the official explanation from the Coast Guard as soon as it was provided."
At a hastily-arranged news conference, Vice Adm. John Currier, the Coast Guard Chief of Staff, said, "We have very, very well developed protocols" for training exercises around the nation's capital. Four vessels, he said, were operating as if they were intercepting an unauthorized boat on the river.
"Part of the protocol in their training is verbalization of gunfire and orders between the boats simulating what we would normally do if we were intercepting a suspect vessel."
"Somebody said, 'bang, bang,' on the radio during the training exercise when the interdiction would have taken place," Currier said. "I don't think our operational commander saw any reason not to train today."