Police were on the lookout for a cell phone, cash and several locks of waist-length, chestnut-coloured hair that three gunmen stole on a Rio bus.
"They got on as the same stop as me, and sat behind me," Mirna Marchetti, 22, told local media.
"Then I felt something pulling my hair. I turned around and asked them to stop it, but they told me this was a robbery.
"Then they cut it really short," she said on Wednesday.
Marchetti was on her way home from her job as a store clerk downtown.
Although she feared reprisals because the thieves could easily figure out where she worked, she spoke out because "a lot of other girls could also become victims of this crime."
Police also warned about the possible return of the crime, popular in the 1990s, because of a similar case late last year.
Police commissioner Valeria de Castro told reporters that the motive was obvious: "They wanted to sell it to beauty salons," which pay 200-300 dollars and more for hair 50 centimeters (20 inches) long, used for extensions and weaves.