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NEW YORK — Pepper-sprayed customers, smash-and-grab looters and bloody scenes in the shopping aisles. How did Black Friday devolve into this?

As reports of shopping-related violence rolled in this week from Los Angeles to New York, experts say a volatile mix of desperate retailers and cutthroat marketing has hyped the traditional post-Thanksgiving sales to increasingly frenzied levels. With stores opening earlier, bargain-obsessed shoppers often are sleep-deprived and short-tempered. Arriving in darkness, they also find themselves vulnerable to savvy parking-lot muggers.

Add in the online-coupon phenomenon, which feeds the psychological hunger for finding impossible bargains, and you've got a recipe for trouble, said Theresa Williams, a marketing professor at Indiana University.

"These are people who should know better and have enough stuff already," Williams said. "What's going to be next year, everybody getting Tasered?"

Across the country, there were signs that tensions had ratcheted up a notch or two, with violence resulting in several instances.

A woman turned herself in to police after allegedly pepper-spraying 20 other customers at a Los Angeles-area Walmart on Thursday in what investigators said was an attempt to get at a crate of Xbox video game consoles. In Kinston, N.C. a security guard also pepper-sprayed customers seeking electronics before the start of a midnight sale.

In New York, crowds reportedly looted a clothing store in Soho. At a Walmart near Phoenix, a man was bloodied while being subdued by police officer on suspicion of shoplifting a video game. There was a shooting outside a store in San Leandro, Calif., shots fired at a mall in Fayetteville, N.C. and a stabbing outside a store in Sacramento, N.Y.

"The difference this year is that instead of a nice sweater you need a bullet proof vest and goggles," said Betty Thomas, 52, who was shopping Saturday with her sisters and a niece at Crabtree Valley Mall in Raleigh, N.C.

Black Friday — named that because it puts retailers "in the black" — has become more intense as companies compete for customers in a weak economy, said Jacob Jacoby, an expert on consumer behavior at New York University.

The idea of luring in customers with a few "doorbuster" deals has long been a staple of the post-Thanksgiving sales. But now stores are opening earlier, and those deals are getting more extreme, he said.

The violence has prompted some analysts to wonder if the sales are worth it, and what solutions might work. In a New York Times column last week, economist Robert Frank proposed slapping a 6 percent sales tax on purchases between 6 p.m. on Thanksgiving and 6 a.m. on Friday in an attempt to stop the "arms race" of earlier and earlier sales.

Next up is Cyber Monday, when online retailers put their wares on sale. But on Saturday many shoppers said they still prefer buying at the big stores, despite the frenzy.

Thomas said she likes the time with her sisters and the hustle of the mall too much to stay home and just shop online.

To her, the more pressing problem was that the Thanksgiving weekend sales didn't seem very good.

"If I'm going to get shot, at least let me get a good deal," she said.