"A preliminary investigation into aggravated child rape has been launched," prosecutor Mikael Karlsson told the Swedish newspaper Expressen.
The suspects were arrested Monday. Karlsson declined to confirm the number or ages of the suspects, but said several were younger than 15, the age of criminal responsibility in Sweden.
The newspaper said the boys videotaped the alleged crime.
The Swedish news agency TT confirmed Tuesday that police have the recording of the incident, and that it constitutes the bulk of the evidence, thus far, against the boys.
Because of the nature of the incident, few details are emerging, but the alleged crime is believed to have occurred in a Stockholm suburb in late February or early March, the news agency said.
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