“Whoever did it knew exactly what they wanted,” said Lisa Amosu, whose salon was burglarized.

The thieves pulled the iron bars out of the windows, outsmarted the motion detector that would have triggered a burglar alarm and did not give the safe or cash register a second look.

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Instead they went straight for what was most valuable: human hair. By the time the bandits at the My Trendy Place salon in Houston were finished, they had stolen $150,000 worth of the shop’s most prized type, used for silky extensions.

The break-in was part of a recent trend of thefts, some involving violence, of a seemingly plentiful material. During the past two months alone, robbers in quest of human hair have killed a beauty shop supplier in Michigan and carried out heists nationwide in which they have made off with tens of thousands of dollars of hair at a time.

“I heard about it from a couple of different supply companies and customers who said: ‘Guard your inventory. There’s a rash of this going on,’ ” said Lisa Amosu, the owner of My Trendy Place. “Whoever did it knew exactly what they wanted. They didn’t even bother with the synthetic hair.”

Once stolen, the hair is typically sold on the street or on the Internet, including eBay, shop owners and the police say.

The most expensive hair type — and the one in highest demand by thieves and paying customers alike — is remy hair, which unlike most other varieties is sold with its outermost cuticle layer intact. This allows it to look more natural and to last longer without tangling. Remy hair from Indian women is the most popular.

But remy hair extensions can cost as much as $200 per package and the average person requires at least two packages. Hundreds of dollars more, and at times thousands, are spent at hair salons to have the extensions attached, often by sewing.

In addition to the $150,000 Houston robbery this month, thieves have recently taken $10,000 in hair from a San Diego shop; $85,000 from a business in Missouri City, Tex.; $10,000 from a shop in Dearborn, Mich.; and $60,000 from a business in San Leandro, Calif. All the values were provided by the storeowners.

Law enforcement officials have been perplexed by the sudden increase in the thefts of hair and the violence that has accompanied some. Some agencies say they had been unaware of the trend before, and others are still learning about it.

“That’s the first I’ve heard of it,” said Denise Ballew, a spokeswoman for the F.B.I., who oversees data related to property crimes.

One indication of how quickly the focus of some thieves has shifted to high-end hair is the experience of the Beauty One hair supply store in Chicago: two years ago, thieves went after the store’s cash, but last month, they bypassed the register altogether and took just the hair, which was valued at $90,000.

Detective Vito Ferro of the Chicago Police Department, who is investigating the April 24 robbery, said some recent hair thefts in the city appeared to be the work of people sophisticated enough to have taken custom orders.

“It’s like someone says, ‘I’m looking for a 1992 Cadillac Eldorado,’ and so you go out looking for that car,” Detective Ferro said.

Surveillance cameras outside the Beauty One shop showed bandits using a crowbar and sledgehammer to pry open dead bolts and then loading boxes of hair into a van.

In recent weeks, packages of hair that may have sold for $80 or $100 retail have sold for as little as $25 out of car trunks in the Bay Area, Los Angeles, Philadelphia and Houston, the authorities said. Hair can be sold at the same types of beauty salons and supply shops that are being robbed.

“They’re selling it to stylists who work out of their house, they’re selling it on the street, they’re selling it out of the car,” said Ms. Amosu of My Trendy Place. “People who don’t want to pay the prices will buy it from the hustle man. It’s like the bootleg DVDs and the fake purses. But this is a quality product.”

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Not long ago, hip-hop songs and black comedians belittled women who wore extensions and weaves. No longer. It is a style grown in popularity that transcends race and celebrity adherence. The market for human hair also includes cancer patients.

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Prices have risen substantially as the quality of hair and the rarity of the most popular hair has increased. Remy hair from India usually comes from women who have their heads shaved as a sign of having mastered their egos.

Neal Lester, an English professor at Arizona State University who has written on the race and gender politics of hair, said the growing demand for human hair extensions and the high prices had made thefts inevitable.

“It’s sort of a sign of the times,” Dr. Lester said. “Folks are being entrepreneurial, and weaves and hair extensions are expensive, so it’s not surprising that people sell hair the way they sell things on Canal Street, like knock-off purses.”

But with the increased profits has come violence, the police say.

In Dearborn, Mich., Jay Shin, the owner of Sunrise Beauty Supply, was killed during a holdup on March 15 by gunmen who stole 80 packages of hair extensions worth about $10,000. Two young men have been arrested.

Assaults have been reported even when only a small amount of hair is involved. In West Palm Beach, Fla., a 16-year-old girl sprayed a clerk with pepper spray last year as she made off with extensions. And in Lawton, Okla., the police said a customer who ran out of a store with extensions tried to escape with the store owner clinging to the hood of her car.

The threat of theft has prompted salons and beauty supply shops to hire security guards, install bulletproof glass partitions and even require patrons to show identification before they are allowed into back rooms to choose their hair.

But surveillance cameras and an expensive alarm system did not prevent thieves from snatching the inventory at Hair Divas Distributors, a beauty shop in San Leandro, Calif., that was robbed of $60,000 in hair last month.

Thieves skipped flat-screen televisions, a digital camera and the cash register, said Ann Davis, the owner. “They went for all my longer pieces, my most expensive stuff,” Ms. Davis said.

She said illegal hair was being sold everywhere, including by people who have come to her shop offering it to customers and by people who have tried to sell it to her on the street.

“ ‘Yo, I got some hair,’ ” Ms. Davis said, imitating the come-on.

“This is not O.K.,” she added. “I’m a little fearful.