Police said the suspect grabbed the girl from behind when she was at an entrance gate at the apartment complex. He assaulted the girl, then fled south on Balboa.
The girl was able to give police enough information for a composite sketch.
The suspect is described as a Hispanic man, about 30 to 40 years of age, about 5 feet, 6 inches tall, and has a thin build.
Police say the 12-year-old girl did not know the suspect.
"It was quite brutal and we would really like to get this suspect identified and off the street," said Lt. Greg Doyle.
Anyone with information is urged to call the LAPD at (877) LAPD-24-7.
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