On the night after the deadliest shooting spree in American history, the nation’s most popular TV networks weren’t covering the grim news during their prime-time hours. Instead, it was escapism as usual: ABC had “Dancing With the Stars” and “The Bachelor,” Fox carried “24,” CBS stuck with “Two and a Half Men,” and NBC showed “Deal or No Deal.”

Which raises a question: Just how big does a story have to be these days to get the broadcast networks to pay attention during their most watched hours?

A few news events in recent memory have commanded “wall-to-wall” coverage on ABC, CBS, Fox and NBC. The Big Four pre-empted their evening entertainment shows in the immediate aftermath of Hurricane Katrina in August 2005 and after the deaths of Pope John Paul II and former President Ronald Reagan. And, of course, the events of Sept. 11, 2001, merited almost a full week of nonstop network coverage, rivaling any other event in the history of television.

But Monday’s virtual non-coverage in prime time – only NBC pre-empted reruns of “My Name Is Earl” for an hour-long report at 10 p.m. – fits a recent historical pattern. More than a decade ago, faced with declining audiences and the choices of airing more profitable sitcoms and dramas, the networks began to cut back on coverage of the political conventions, presidential addresses and election-night results. More often than not, entertainment, not news, rules.

“Everyone can appreciate the business pressure that the networks are under, but when did they (start) ceding their responsibility to cover these stories?” said Tom Kunkel, dean of the University of Maryland’s journalism school. “It does kind of make you wonder how big a bloodbath there has to be warrant their attention in prime time. How bad does it have to be to supplant ‘Dancing With the Stars’?”

Fox did make one concession to the unfolding tragedy: It has decided to pull an episode of “Bones” on Wednesday night in which a college basketball star is discovered dead on campus. Fox will rerun an earlier episode.

TV, and the networks themselves, surely did not ignore the unfolding tragedy. CBS, ABC and NBC broke into their regular programming early Monday and began issuing periodic updates. The all-news cable networks, including the network-owned MSNBC and Fox News, carried nonstop coverage.

Monday, none of the networks’ representatives would comment directly on why their news coverage did not extend to prime time.

Tom Rosenstiel, director of Washington’s nonprofit Project for Excellence in Journalism, said the traditional broadcast networks, and the prime-time hours (8 to 11 p.m.), have special status. About 45 percent of Americans are watching one of the four networks during those hours, a share that typically dwarfs the next 10 cable or broadcast networks combined.

“They’d rather run reruns than pre-empt their regular programming,” Rosenstiel said. “It’s not a surprise, but it is unfortunate. If the networks have lost their role as arbiters of what’s significant in our culture, then they’ve been complicit in that loss.”

The networks defended their coverage Monday, saying they offered special reports throughout the day, plus more information on their Web sites.

ABC, for example, said it expanded its “World News” broadcast by five minutes to offer more news about the shootings, and expanded “Nightline” from its usual half-hour to a full hour. It was to have more coverage on its morning program, “Good Morning America.”

“No one watching ABC today could conclude that it wasn’t a big, huge, important story,” said ABC News spokesman Jeffrey Schneider. “Obviously, it’s very important. But at the same time, we don’t want to be endlessly repeating the same information.”

CBS News spokeswoman Sandy Genelius called the shootings “obviously a huge story, and we will be covering it aggressively. CBS viewers will be very well informed.” CBS expanded its evening news to a full hour Monday.

Both CBS and ABC were to devote the 10-to-11 p.m. hour of their evening schedules Tuesday night to the aftermath of the shooting. CBS normally airs “48 Hours Mystery,” examinations of real-life murders, in that time period; ABC was to pre-empt “Boston Legal.”

Fox, which has the smallest network-news division, had no plans for any prime-time pre-emption. “It’s a fluid situation and we’re monitoring it,” Fox spokesman Scott Grogin said.

All of which disappoints media critics such as Kunkel. “We’re accustomed to 24/7 cable news,” he said, “but we as Americans are still enculturated that when a story reaches a certain scale it reaches into network scale. When it’s on the networks, it says, ‘Hey, folks, we as a nation need to pay attention to this.’ When it’s in prime time, it says we as a country are stepping back and having a national conversation about it.”