Perhaps this was not the best day to be conduct a training exercise on the Potomac River, not far from the Pentagon which was attacked eight years ago this morning. Still, when reports began to surface that a suspicious vessel had been spotted on the Potomac, CNN went into high gear. They got their Homeland Security, Pentagon and White House correspondents on the case, even discussing the situation with former Homeland Security adviser, now CNN contributor, Fran Townsend to help with their coverage -- coverage that has since drawn the ire of the White House.

At 10:05am CNN interrupted live remembrances of the 9/11 memorials to report on the situation. "We have seen at least one boat try and challenge" a security zone set up around the Pentagon, said Homeland Security correspondent Jeanne Meserve. "A cat and mouse game going on there," she added. Shots were fired, they claimed.

While the observations may have been accurate, it was all a drill. A fact CNN would not report until 10:32am. There were no shots fired. Live or blank. The report claiming 10 rounds had been fired came from Coast Guard audio transmissions CNN aired -- again, all part of the exercises.

At an off-camera, on-the-record briefing as the truth was being revealed, CNN White House correspondent Elaine Quijano asked White House Press Secretary Robert Gibbs whether the public should have been told about a training exercise happening on the eighth anniversary of the 9/11 attacks. "If we set aside the media coverage, would you be asking this question? My only caution would be that before we report things like this, checking would be good," Gibbs said.

NBC News Chief White House correspondent Chuck Todd took to his Facebook page to call out CNN, writing, "Weird to hear others blame the government for their own bad reporting," and later, "Not only were there no gunshots, CNN reports what they heard on the radio scanner was someone SAYING 'bang bang; they didn't HEAR shots"

While CNN was in full breaking news mode, MSNBC remained with a re-broadcast of the "Today" show from 9/11/01. MSNBC spokesman Jeremy Gaines tells TVNewser, "We chose to do what is always prudent in these situations and gather the facts. Once we had the facts, we broke into our 9/11 tape and reported the news of the training exercise." That happened at 10:37am.

Fox News, meanwhile, began reporting on the incident at 10:12am. At 10:23 they began speculating that it might be an exercise. By 10:30 they'd confirmed it. TVNewser has obtained notes from FNC's urgent queue including details from Bret Baier and Catherine Herridge in Washington, before information got on the air:

10:18am - "Caution in reporting per baier -- it could be that if a boat wasn't responding warning shots could be fired in the air or near the boat. so we should be careful about saying that the coast guard fired at or on the boat on the potomac."

10:42am Herridge: "Dhs Spox Matt Chandler 'No shots fired on the Potomac. Boats were part of a Coast Guard Exercise. Coast Guard will be taking all inquiries.'"

As for CNN, the network released a lengthy statement late this afternoon explaining their side of it: "CNN staff were monitoring law enforcement activity this morning given the 9/11 anniversary. After hearing a U.S. Coast Guard radio transmission that a boat had breached a security zone on the Potomac River a short distance from the Pentagon where the President had just attended a 9/11 anniversary ceremony, CNN contacted the Coast Guard public affairs office at the agency's headquarters. The Coast Guard spokeswoman said she was unaware of any activity taking place on the Potomac River.

After hearing a further radio transmission about 10 rounds being expended, and after reviewing video of rapid movement by Coast Guard vessels as the President's motorcade crossed the Memorial Bridge, CNN reported the story. Simultaneously, during a second phone call, the Coast Guard spokeswoman informed us that its National Command Center and other command posts knew nothing about any activity in the area.

Given the circumstances, 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."

• Click continued to see CNN's original reports, including Coast Guard audio, and the follow-up reporting...