(CNN) -- As many as 12,000 Puerto Rican schoolchildren, teachers and school administrators are believed to be victims of an identity-theft ring that sold stolen personal documents to illegal immigrants in the mainland United States, according to the FBI.
A federal grand jury this week indicted eight people on charges of identity theft, aggravated identity theft and Social Security fraud.
"In search warrants, we found over 5,000 different types of identification documents -- originals and copies, Social Security cards and birth certificates," FBI spokesman Harry Rodriguez said in a telephone interview Tuesday from San Juan.
"They were selling them as a set; an original birth certificate and Social Security card sold for $150 or more. Copies sold for $40 or more," he said.
Rodriguez called it the biggest case of identity theft in his 15 years with the FBI in Puerto Rico.
The investigation began after a string of school burglaries swept the island in 2007, said Lymarie Llovet-Ayala, a spokeswoman for the U.S. attorney's office.
Authorities believe the suspects broke into about 50 public schools and stole personal files belonging to students, teachers and school administrators. In Puerto Rico, students must provide an original or a copy of their birth certificate to be registered in a school.
"They are children; they don't have credit cards or property so it is much harder to detect identity theft," Llovet-Ayala said. "We are in the process of identifying all the victims."
Llovet-Ayala said authorities have been able to identify buyers of stolen identification materials in Texas, Alaska and California, where ring members mailed documents to different contacts.
The seven men and one woman indicted Monday -- some from Puerto Rico and some from the Dominican Republic -- face up to 15 years in a federal prison, with fines up to $250,000 if they are convicted, she said.
The suspects had their first hearing Tuesday and are being held in a federal prison in Puerto Rico. Authorities are searching for another suspect, she said.
The Puerto Rican suspects will have their bail hearing Wednesday. The Dominican suspects will appear in court April 13. The agency for Immigration and Customs Enforcement will take over the latter group once they have been processed in Puerto Rico.
The U.S. attorney's office has asked potential victims to contact the FBI at 787-754-6000 or 877-324-7577.