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."
In response to a reporter's question, Currier said, "I am not issuing an apology, because, although it's unfortunate that it escalated to this level, what you're seeing here is the result of a normal training exercise."
Coast Guard staff played a recording of the radio traffic during the drill. In it, a crew member on one boat is heard saying, "If you don't slow down, we're going to -- you will be fired upon. Roger, roger midzone."
Currier said the Coast Guard will reevaluate its communication procedures to prevent future misunderstandings.
A Coast Guard public affairs officer said that though weapons are mounted on the boats involved in the exercises, they are not loaded.
According to ABC News Senior White House Correspondent Jake Tapper, White House Press Secretary Robert Gibbs said he was not aware of any heads-up given by the Coast Guard to the White House about the Potomac training exercise and suggested any panic that resulted was the fault of the media, not law enforcement.
Gibbs said the "reporting was based on listening to a police scanner" and wasn't "verified" before being reported. He said media should "check before reporting" such events.
Asked if Sept. 11 was an appropriate day for such a training exercise, Gibbs said, "I tend not to question law enforcement trying to keep the nation safe."
"If anyone was unnecessarily alarmed based on erroneous reporting that denoted shots had been fired, I think everybody is apologetic of that," Gibbs said.
Coast Guard Commandant Thad Allen was at the Pentagon for the Sept. 11 remembrance ceremony this morning. Homeland Security Secretary Janet Napolitano, according to one source, was unaware the exercise was to take place.
About an hour after the scare, the Coast Guard put out a statement reading, "We are still gathering information of how this training event might have been misconstrued as an actual incident. We will conduct a thorough review of this incident."
It continued, "The best way that we in the Coast Guard can remember Sept. 11 and our security obligations to the nation is to be always ready and this requires constant training and exercise. To ensure the appropriate readiness posture we conduct training scenarios across the nation on a daily basis."
Others joined in anger about the anxiety created on a day of mourning.
"The anxiety caused by this situation on such a solemn day is extremely disturbing," Sen. George Voinovich, R-Ohio, who sits on the Senate's Homeland Security Committee, said in a statement. "I look forward to hearing from Secretary Napolitano about the decision-making process leading up to today's events. It sounds very much like the left hand didn't know what the right hand was doing."
Military Families United, an advocacy group, issued a statement as well: "Whomever commissioned this training exercise at the same time and less than a mile away from where the families of the 9/11 victims gathered to mourn should be held accountable. Their actions brought back all of the feelings for victims of 9/11 that they originally experienced 8 years ago today."
ABC News' Brian Hartman, Lisa Stark, Brian Ross, Pierre Thomas and Jack Date contributed to this report.