In one instance, rats215 posted photos and evidence from a shooting victim whose case was handled in a secret grand jury.
In another, the account posted a photo that appeared to have been taken while a witness was testifying in court.
The account has been updated almost daily, and each new post draws dozens of comments and "likes" from readers, with some of the commmenters advocating violence against those who help police. The page has more than 150 photos.
"Post some new rats," one commenter wrote in September. "I needa put a hit out on them."
Said another, earlier this week, "This page is [. . .] perfect."
In one post, the account-holder praises Kaboni Savage - the North Philadelphia drug kingpin sentenced to death in June for 12 murders, including a fireboming that killed four children and two women. The bombing was retaliation for a witness' cooperation with the FBI.
In the same post, rats215 warns witnesses "we will get at you in time."
Tasha Jamerson, a spokeswoman for the District Attorney's Office, said she could not comment on any investigation, but stressed witness intimidation was a "very serious, ongoing problem" that plagues prosecutors every day.
"We work with Philadelphia police to investigate vigorously and thoroughly any attempt to intimidate any witness, to identify perpetrators, and, where appropriate, arrest and prosecute," she wrote by e-mail.
Police learned of the account last week when a 12th District police officer monitoring Twitter spotted photos of a witness and court records from a 2012 attempted shooting, law enforcement officials said. That led police to the Instagram account.
There, they found witness statements from the 19-year-old victim, according to Lt. John Walker of Southwest Detectives. The victim told police he was fired at in summer 2012 because he had testified in a homicide case.
The case was handled by a secret indicting grand jury - a practice authorized last year by the Pennsylvania Supreme Court to protect witnesses and combat low conviction rates in violent crimes.
It remains unclear how rats215 obtained that witness' testimony. The account-holder routinely asks followers to send along paperwork on suspected "rats."
Though witness statements are sometimes available in court records when a case is closed, the names of witnesses and victims are redacted for their protection.
But in copies of statements posted on the account, witnesses are clearly and repeatedly named, sometimes with their photographs attached.
In indicting grand juries, tougher rules limit the disclosure of statements from witnesses and victims. Defense lawyers are instructed not to give their clients copies of such statements. The defense is free to go over the statements with their clients, but they can't make copies for the accused.
Another special rule for these grand jury cases allows prosecutors to wait until two months before trial to turn over statements.
The trial in the grand jury case is not scheduled to begin until March.
Veteran law enforcement officials and prosecutors describe cases where victims' statements have been posted in barbershops, on neighborhood light poles, and even mailed to witnesses' homes.
Now those statements are just as likely to end up on social-media sites like Facebook, Twitter, and Instagram, where untold numbers of users can see and share them.
"People feel they can hide behind a name," Walker said. "It's a challenge we're facing and a challenge we're attempting to work through."
Police investigators are working with the department's criminal intelligence unit to find the account-holder, he said, adding:
"These actions shoot an arrow through the heart of the criminal justice system, placing victims and witnesses at risk. [Accounts like rats215] should be voluntarily removed by the host of such sites."
Instagram representatives said Thursday night they were looking into the account. By 8 p.m. Thursday, the site was inaccessible.
Online witness intimidation has become more prevalent in recent years, leading Justice Department officials to call for federal courts to make it harder for the public to access court documents online.
In one case, described in a 2007 New York Times article, a Philadelphia witness was forced to move after material related to his case was posted on a website dedicated to exposing criminal informants and witnesses around the country.
Inquirer staff writer Craig R. McCoy contributed to this article