Coast Guard officials say they were conducting a training exercise in the Potomac River moments before President Barack Obama's motorcade  crossed en route to the Pentagon for a Sept. 11 commemoration.

Media reports suggested at first that shots had been fired on a suspicious boat. But Coast Guard officials denied that those reports, saying instead that radio chatter had accidentally picked up references to shots fired.

As part of its exercise Friday, the Coast Guard aired simulated instructions to participants to fire 10 rounds. Coast Guard spokesman John Edwards said there were no shots actually fired and there were no suspicious boats.

A law enforcement official said federal agents scrambled to the river scene after the initial reports. Local FBI offices had not been told ahead of time about the exercise.

All departures from Reagan National Airport were stopped shortly after 10 a.m. as a precaution, Federal Aviation Administration spokeswoman Diane Spitaliere said. The airport borders the Potomac River.

The incident happened as the nation commemorated the eighth anniversary of the Sept. 11 terrorist attacks.