"I don't care how old you are, if you intimidate a witness in this city I'm going to come after you," District Attorney Seth Williams said.
Anderson's social media activity first caught investigators' attention last month when a police officer monitoring Twitter spotted photos of a victim in a 2012 attempted shooting posted on Anderson's account alongside statements the victim made to police.
The statements were not public record and had made their way out of a secret grand jury proceeding - a process designed as a bulwark against rampant witness intimidation in city courts.
"The actions of this teenager could have lasting repercussions on untold cases here in Philadelphia," Williams said. "Witness intimidation has reached near epidemic levels and we are very serious about not only stopping it, but also prosecuting the criminals who are engaging in these despicable actions to the fullest extent of the law."
Police reports related to the 2012 shooting were also posted on Anderson's Twitter account, officials said. Beneath one, Anderson had allegedly written, "Expose All Rats."
About 10 days later, the same photos appeared on the Instagram account with the rats 215 handle. For months, that account had been identifying witnesses in dozens of cases by posting photos, police statements, and testimony.
Anderson's mother, who declined to give her name, answered the door Tuesday night at her home. She described her son as a "good kid" and said he did not get into any trouble. She said he was in 11th grade, boxed at a local gym, and wants to go into the Army after graduation. She said she did not want to speak about his arrest.
Like the statements Anderson allegedly posted on his Twitter account, many of the documents on rats 215 are not public records. Authorities are investigating whether rats 215, which was taken down Thursday night, was an act of witness intimidation as well.
Law enforcement sources told The Inquirer on Friday that they suspected Anderson could be behind the Instagram account, since some of the photos posted there also appeared on his Twitter page.
On Tuesday, those same officials said Anderson's connection to the page remains unclear.
Anderson was being held Tuesday night pending an arraignment and bail hearing. Tanya Little, a police spokeswoman, said it was unclear whether he would be charged as a minor or an adult.
Police have submitted a warrant to compel Instagram to deliver the account owner's identity, the sources said. The department's Criminal Intelligence Unit is handling the investigation into rats 215.
The photos Anderson is accused of posting on Twitter had to do with the attempted shooting of a 19-year-old Southwest Philadelphia man who told police he was targeted because he had testified in a homicide case. Police say Anderson knew the defendant.
Defense attorneys are allowed to provide their clients with witness statements in most cases as a constitutional right. But in grand jury cases, defense attorneys are instructed not to give their clients copies of statements.
One official with knowledge of the case posted on Anderson's account said evidence from the grand jury case had been made public after a series of inadvertent errors by a prosecutor and a defense attorney.
Police say they are still not sure how Anderson could have obtained the information.