BOSTON -- An incident on the Potomac River in Washington, D.C., sparked fears Friday as the nation paused to honor the victims of the Sept. 11, 2001 terrorist attacks.

The U.S. Coast Guard was conducting training on the river at about 10:15 a.m. as President Barack Obama's motorcade left a ceremony honoring the victims.

Early media reports indicated that shots were fired on a suspicious boat on the river that entered a restricted area of the river. Coast Guard spokesman John Edwards said officials aired simulated instructions during the training to fire 10 rounds, but no shots were actually fired.

Coast Guard boats were operating near 14th Street and Memorial Bridges.

"Whether or not these were the same boats using the marine radio frequency used for training purposes has not yet been confirmed," the Coast Guard said in a statement.

Shortly before the incident, Obama attended a ceremony at the Pentagon honoring the victims of the attacks. The president has since returned to the White House.

"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," the Coast Guard said.

Earlier this year, a controversial low-altitude New York flyover by a 747 plane used as Air Force One sent some New Yorkers into a panic. Officials said that the April 27 flyover was a training mission and photo shoot. In that incident, an F-16 accompanied the plane.

NewsCenter 5 and TheBostonChannel.com will have more information when it becomes available.