Will Hart/NBC Christopher Meloni portrays detective Elliot Stabler and Mariska Hargitay is detective Olivia Benson on "Law and Order: Special Victims Unit," a show that is bucking the declining ratings trend. Click photo for larger image.

The nation's crime wave -- the one on TV, that is -- may have peaked.

Ratings for some of prime time's top crime shows, such as NBC's "Law & Order" and CBS's "CSI" franchise, have slumped this fall, raising suspicions about the genre that sustained network drama departments for years before "Desperate Housewives" and "Lost" came along.

Executives say that so-called procedurals, which use the crime-solving process to advance plots, still have plenty of life. But there are growing signs of a shakeout on the way.

"It started to become a little bit overstated that, 'Well, a procedural will work,' " NBC Entertainment President Kevin Reilly said this week. But "if you bring on a generic show, odds are it's not going to do particularly well."

Added Nina Tassler, president of CBS Entertainment: "The whole [television] landscape is more competitive. You can't deny that."

Consider, as "Dragnet's" Detective Joe Friday might say, just the facts about two high-profile Wednesday shows:

"Law & Order," now in its 16th season and the forerunner of virtually all of today's top procedurals, has seen a 16 percent slide among the advertiser-friendly young-adult audience compared with last year, according to figures from Nielsen Media Research. (The show is down a more modest 5 percent in total viewers, to 12.9 million.) "CSI: New York," which like "Law & Order" airs at 10 p.m., has shed an alarming 29 percent of its young-adult audience and 23 percent of its total viewers (to 14.3 million), although it's still the most-watched program in the time slot. Both shows have been heavily affected by ABC's new thriller "Invasion."

Any declines among young adults are especially worrisome to TV executives because those viewers are considered hard to win back, advertisers pay premium rates to reach them and their viewing habits are often predictive of general audience trends, researchers say.

Crime is paying smaller dividends on other nights, too. CBS's "CSI: Miami" and NBC's "Law & Order: Criminal Intent" have each slipped 19 percent in viewers ages 18 to 49 this season. Even "CSI: Crime Scene Investigation," the smash hit that unleashed the forensics craze back in 2000, has stopped increasing its total audience and is down 6 percent in young adults, though the show remains No. 1 in the ratings. The drop-offs are especially noticeable when compared with overall network performance: No. 1-ranked CBS is up 1 percent this season in total viewers, to 12.9 million, followed by ABC (11 million, up 12 percent), NBC (9.5 million, down 5 percent) and Fox (8.9 million, down 20 percent).

Among the few crime shows bucking the trend are two spinoffs, NBC's "Law & Order: Special Victims Unit" and CBS's "NCIS" (which followed "JAG"), which have both posted gains across the board this season.

But generally, the outlook for procedurals is one of decline, and many TV veterans say there's no mystery why: In addition to the competition from new hits like "Lost," there are simply too many look-alike crime shows.

Of course, me-too programming is a time-honored TV tradition. The surprise success of ABC's "Lost" last year brought a spate of supernatural-themed shows this season. CBS's "Survivor" series spurred a reality gold rush that is only now abating. What makes the procedural dominance unusual is that it's lasted for so long and spawned so many shows.

Blame "television's unalloyed penchant for wretched excess and cloning," said Tim Brooks, a TV historian and executive vice president of research at Lifetime Entertainment. The procedural genre, he added, is "beginning to show signs of wear."

Mysteries and crime shows of various stripes occupy just under half of CBS' weekly schedule, including a two-hour block of repeats dubbed "Crimetime Saturday." Add the three "Law & Order" shows on NBC and there were already very few time slots for any new procedural that would not put it in instant competition with a similar show on another network. And yet this season has somehow brought even more.

CBS recently ordered more episodes of "Criminal Minds," a profiler show starring Mandy Patinkin that has delivered respectable if not enormous ratings. Fox has gotten into the act with a modestly successful forensics show called "Bones."

CBS's Tassler disputes the idea that the genre is overcrowded, arguing that each of the shows has certain story and character elements that set it apart.

"The only thing I take a little bit of offense with is to sort of lump them all into one thing," she said. "We have viewers that are really attracted to shows that have a crime-solving, mystery or procedural element. ... The fact that 'Criminal Minds' is certainly showing signs of growth [proves] that our audiences are still responding to shows with those kinds of elements."

Others take a different view.

"There are too many procedurals," said "Law & Order" creator and executive producer Dick Wolf. "Most of these shows will go away over the next couple of years." He added: "I certainly don't think any of the 'Law & Orders' or 'CSIs' fall into that category."

Wolf is doing everything he can to make sure his shows aren't among the dropouts. He is still seething that NBC canceled a third spinoff, "Law & Order: Trial by Jury," after 12 episodes last spring. He has long said he wants the original "Law & Order" to stay on the air at least 21 years, thus beating CBS's western "Gunsmoke" as TV's longest-running drama.

NBC has strong incentives to help him achieve that goal. The network acquired the studio rights to the entire "Law & Order" franchise as part of its merger with Universal, and repeats earn high ratings on NBC's cable networks USA and Bravo, as well as on Time Warner's TNT. NBC estimates that the three shows reach 100 million viewers per month, and its total ad tally for the franchise amounts to a reported $1 billion annually.

"Law & Order," though, may be a victim of its own success, because it inspired the creation of a host of other procedurals that now threaten its existence. The original show is in many ways a throwback to Jack Webb's pioneering cop drama "Dragnet" -- a pure procedural that generally ignores what detectives and prosecutors do in their off-duty hours.

Wolf says NBC has grown "overly reliant" on the "Law & Order" franchise. At the same time, he remains unhappy about the fate of "Trial by Jury," which focused on the preparation behind criminal trials.

"It never should have been canceled," he said.

But Wolf doesn't have too much cause to worry. NBC is already working with him on another series. It's -- you guessed it -- a procedural about assistant district attorneys called "Conviction."

Viewers may find the show familiar in more ways than one. As Reilly noted, the new show will use "Trial by Jury's" old sets.