In more than three decades of hanging around TV stations and studios, I have never seen anything like what happened on NBC Nightly News Tuesday night.

Brian Williams was just introducing the lead story of the newscast about the American Airlines bankruptcy filing when a fire alarm sounded in the NBC studio, followed by a voice announcement.

Watch video of the alarm sounding during the story:

The NBC Nightly News team tweeted about the alarm.

Williams -- a former volunteer firefighter as a reader reminds us -- kept his cool, offered an easy chuckle and assured viewers that it was just an alarm and there was no danger. Surely the problem would go away. It didn't. He tossed to Andrea Mitchell.

Within minutes, Brian Williams and NBC Nightly news were trending on Twitter.

Viewers started reacting, amazed that Williams could keep going. Why didn't somebody smash the alarm, they asked on Twitter?

The alarm buzzed on, through the Herman Cain story, through the lead-in to a story about the siege at the British Embassy in Iran.

Williams took a commercial break. When he came back, so did the alarm. Each time, Williams did exactly what he should do, he kept his cool, reminded viewers everything was OK and kept going, even while his voice was overwhelmed by the sound as he introduced a piece on Mexican drug wars.

Three-quarters of the way through the program, the alarm fell silent.

How did it happen?

My Twitter followers who work in television tell me that depending on where you live, fire codes may require alarms, even in studios. And sometimes the alarms just do what alarms do, they go off.

Kingsley Smith is the News Director at KTTV FOX 11 in Los Angeles and sent me a picture of a sensor in the ceiling of his station's studio.

Samara Sodos, a reporter at WFLA in Tampa, remembers when this happened at her station, too.

NBC's PR spokeswoman tweeted an explanation:

"With thanks for your cooperation," Williams said as he closed the program, "We hope to get all quiet and see you here tomorrow night."

West Coast viewers missed the excitement as NBC re-did the program live for that feed. NBC posted a final tweet to viewers.

Back in February, NBC San Diego had a fire alarm go off during a live broadcast. The station switched to MSNBC while it sorted out the problem.

Also earlier this year, a fire alarm went off during a live BBC broadcast.