The 17-year-old was taken into custody Tuesday in connection with the now-shuttered anonymous Instagram account, said Officer Tanya Little, a police spokeswoman.
Little said the boy is alleged to have been the owner of the "rats215" account - a reference to Philadelphia's area code - that posted affidavits and photos of victims and complainants in criminal cases.
"There was a lot of information there ... a lot of information that was not for public viewing," Little said Wednesday.
The Philadelphia Inquirer reported earlier that the account had posted material identifying more than 30 witnesses since February. The newspaper said the account had nearly 7,900 followers and had more than 150 photos, many drawing dozens of comments and likes.
Prosecutors said they would seek to try the teenager as an adult in connection with information posted about a shooting last year in southwest Philadelphia.
"The actions of this teenager could have lasting repercussions on untold cases here in Philadelphia," District Attorney Seth Williams said in a statement, saying witness intimidation had reached "near-epidemic levels" and vowing to prosecute those responsible.
"I don't care how old you are, if you intimidate a witness in this city I'm going to come after you," he said.
Little said the investigation, which started Oct. 24, was continuing. Although the teenager is accused of having owned the account that posted the material, "you have to think about ... where did it come from?" she said. "That's the question that has to be answered."
Authorities have called witness intimidation a serious problem in Philadelphia, with people arrested daily in the city's criminal courts building for taking photos of witnesses, victims' families or judges and posting the images online, said Tasha Jamerson, a spokeswoman for the district attorney's office.
Law enforcement officials have said they have long seen victim's statements posted in public places or sent to the home of witnesses, but such material is now winding up on Facebook, Twitter and Instagram.
Instagram hasn't said whether it took down the account. It says community guidelines bar content that bullies or harasses and that users are encouraged to report it.
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