WASHINGTON - The U.S. Coast Guard set off a security scare on Friday with a routine training exercise in the heart of the U.S. capital on the anniversary of the September 11 attacks.

Police and federal law enforcement officials said the incident on the Potomac River near the Pentagon was a training exercise and no shots were fired despite early reports that several rounds had been shot at a suspicious boat.

Coast Guard Vice Admiral John Currier said the reports were based on overheard radio calls made on an open training frequency, including authorities verbally simulating gunfire. Somebody said bang, bang on the radio, he said.

This was a routine, low profile, normal training exercise, he told a news conference. The radio intercept generated intense media coverage and interest, justifiably so, but I think what happened was we saw this spiral out of control.

The scare came as President Barack Obama had just returned from a September 11 memorial at the Pentagon on the south side of the river in Virginia. During the 2001 attacks, a jetliner slammed into the side of the Pentagon killing 184 people.

Because the exercise was routine, Currier said there was no notice to federal, state or local authorities. He defended the Coast Guard's conduct and said they do such exercises about four times a week in the Washington area.

Departures from Washington National Airport were halted for about 20 minutes because of the incident, according to the Federal Aviation Administration. Arrivals were not affected at the airport which also borders the Potomac River.

White House spokesman Robert Gibbs offered support to the Coast Guard and chastised reporters for not verifying the information before airing the story.

Best I can tell, there was reporting based on listening to a police scanner that was not verified. Then it was on television and now we've raced back to find out it was a training exercise, he said. So it appears a lot of this could have been avoided.

The incident is the latest by the military to prompt a scare. Earlier this year, one of the jumbo jets used as Air Force One to carry the president buzzed the New York City skyline for a photo shoot, sparking fears among some New Yorkers who recalled airliners crashing into the World Trade Center twin towers in the September 11 attacks.

The military admitted it failed to properly coordinate with relevant authorities about the flyover, which led a White House military aide who authorized the flyover to resign.

The Coast Guard promised to review how and when the training exercise was conducted and their response to the reports.

Sandra Rus, a bus driver visiting Washington from Georgia, said she was not bothered that a training exercise occurred on the September 11 anniversary. Someone might be upset but I'm not. I just feel more safe, she said.

In all, about 3,000 people were killed in the September 11 attacks in which Al Qaeda hijackers crashed two planes into the World Trade Center and a third into the Pentagon and a fourth crashed into a field in Pennsylvania.

(additional reporting by David Alexander, James Vicini, Deborah Charles and Christopher Doering, editing by Jackie Frank and Vicki Allen)

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