WASHINGTON — - The Coast Guard promised a full investigation Friday after a training exercise on the Potomac River grounded flights at Reagan National Airport amid media-fueled fears of another Sept. 11 incident.

False TV reports about machine gun-equipped Coast Guard vessels firing on a suspicious craft in a security zone not far from a 9/11 commemoration at the Pentagon prompted sharp criticism of the government for staging the drill at such a sensitive moment. In turn, the White House pushed back against cable TV networks for erroneous "breaking news" reports.

Vice Adm. John P. Currier said the Coast Guard would conduct "a top-to-bottom review" of the exercise, which had been authorized by its commander in Baltimore.

Democratic Rep. Elijah E. Cummings, chairman of a House subcommittee that oversees the Coast Guard, said he could not understand why the service would be simulating an incident on the Potomac "when the nation was on high alert, when families of 9/11 victims were nearby and when the president was either in the vicinity or had just left," without at least notifying relevant authorities in advance.

The Baltimore congressman said that he was listening, with alarm, to CNN's live coverage on his car's satellite radio, "and I was wondering, 'Do we have another terrorist-type situation where they want to commemorate 9/11?' " He said his committee would look into the incident.

During the exercise, which began about 9:30 a.m., four 25-foot patrol craft churned up the surface of the river while a Coast Guard helicopter zoomed low overhead.

Unaware that it was only a simulated confrontation, CNN, which was monitoring police radio frequencies, relayed the Coast Guard conversations to its audience. Within minutes, Reuters news agency and Fox News Channel picked up the CNN story.

FBI and local police vehicles rushed to the scene, apparently in response to the CNN report. The Federal Aviation Administration halted takeoffs at National Airport, whose flight paths follow the river, for about 20 minutes.

White House spokesman Robert Gibbs harshly criticized the news coverage.

"Before we report things like this, checking would be good," he said.

CNN said it called the Coast Guard public affairs office in Washington before airing its initial report and was told by a spokeswoman that she was unaware of anything happening on the Potomac. It took about 30 minutes before the network let viewers know that it had raised a false alarm.

"Given the circumstances, it would have been irresponsible not to report on what we were hearing and seeing," the network said in a statement.

At a hastily called noon news conference, Currier, the Coast Guard's chief of staff, said there had been "really no reason" to provide any advance warning to other law enforcement agencies or the public about the drill, which he called an "everyday exercise."

Public advisories are routinely issued in advance about exercises involving military flights over the Washington area, to help avoid unnecessary confusion.

President Barack Obama, who attended the Pentagon ceremony with his wife, Michelle, crossed a Potomac bridge close to the scene around the time that the exercise was under way.

Currier said the exercise had been authorized by the Coast Guard unit in Baltimore that is responsible for operations in the national capital region. The Baltimore office is commanded by Captain Mark P. O'Malley, a 28-year Coast Guard veteran. A call to O'Malley's office was not returned.

The incident "sounds very much like the left hand didn't know what the right hand was doing," Sen. George V. Voinovich of Ohio said in a statement.

Voinovich, senior Republican on a homeland security subcommittee, said that the "anxiety caused by this situation on such a solemn day is extremely disturbing."

A military families' advocacy group that has been critical of Obama administration policies, including its handling of detainees at Guantanamo Bay, issued a statement deploring the training exercise as "absolutely inexcusable." It called on the government to hold accountable those responsible for authorizing the Coast Guard training.

"This training exercise not only caused unwarranted stress for [9/11] families but it was a distraction from the purpose of today," the group Military Families United said in a prepared statement.

The incident had echoes of a White House photo shoot last spring, in which two fighter planes and an Air Force jumbo jet from the presidential fleet flew low over New York harbor, panicking residents who feared a repeat of the 9/11 attack. A White House aide who authorized the flight subsequently resigned.