Monday’s  Boston Marathon was so surprising and horrific to the American public that CBS, ABC and NBC all expanded their nightly newscasts from the usual half an hour – no mean feat after spending hours on air covering developments as they happened.

But NBC decided it had more stories to tell, and, unlike its broadcast brethren, went back on the air at 10 p.m. — during primetime —  to give viewers a deeper dive into the day’s still-unfolding events. “There were new developments continuing and it was something we thought could provide a really valuable public service,” said Alexandra Wallace, a senior vice president at NBC News. “That’s something we think about a lot.”

Leaving the air shortly after President Obama made some public remarks gave the network’s news operation time to develop approximately six to eight produced segments, Wallace said, ranging from a look at the ongoing investigation to the Boston Marathon’s odd connection to the recent shooting tragedy in Newtown, Conn. The time off the air also gave NBC time to find informed sources who could serve as guests on a 10 p.m. broadcast.

A decision was made sometime in the afternoon to air more coverage during primetime, Wallace said. “I know that we all felt this warranted an hour of news, and we had a great conversation with the West Coast and they felt exactly the same way.”

Rival news divisions also kept coverage alive. ABC News expanded its “World News With Diane Sawyer” to an hour Monday and will air another expanded edition Tuesday evening. CBS expanded its “CBS Evening News With Scott Pelley” to 90 minutes, and featured an interview with New York City Police Commissioner Ray Kelly on “CBS This Morning” Tuesday.

“We had no idea” NBC would be the only broadcaster offering additional coverage in prime time, Wallace said.

The decision to break into primetime can be a frenzied one, and at NBC, a number of executives were involved in working out logistics, Wallace said. News broadcasts aren’t judged by advertisers on the same sorts of merits as regular primetime entertainment; sponsors look for the number of viewers between ages 18 and 34 for entertainment fare and viewers between 25 and 54 for news programming.  Introduction of a newscast into Monday night’s lineup could be scrutinized as sponsors try to make apples-to-apples comparisons with previous Mondays.

In the end, NBC decided to pre-empt the drama “Revolution,” which has seen its ratings tumble in recent weeks, while keeping intact its most reliable primetime offering, “The Voice.” Viewer interest in the Boston news and “The Voice” lead-in combined to give the night’s special news programming the second-highest ratings among 18 to 49’s on Monday night, per Nielsen.  The NBC coverage was watched by more viewers than anything else on Monday night except “The Voice” and ABC’s “Dancing With the Stars” and “Castle.”

NBC News is ready to tackle a primetime assignment again if circumstances dictate, Wallace said. “It’s a lot of work and long hours,” she said, but “we would do it again tonight if it warranted.”