WITH PLENTY of preening self-portraits and pointless chatter with friends, 17-year-old Nasheen K. Anderson's Twitter feed at first looks like that of any typical teenager.

But it doesn't take long before you find some jaw-dropping stuff. In one photo Anderson posted, a cat rests its head on a man's erect penis. Another shows a woman squirting milk from her naked breast into a coffee mug. Others show extreme close-ups of women's naked genitalia, Anderson flashing gang signs or showing a handgun jammed beneath a mattress, and a "family photo shoot" of a smiling woman pointing a handgun at four beaming school-age children.

But it wasn't these images that caught the attention of Philadelphia police last month. Instead, they found secret court documents and photos that outed witnesses of two shootings in January and June 2012 and a 2007 homicide. "EXPOSE ALL RATS," the caption beneath one reads.

After confirming that the account was Anderson's, police on Tuesday arrested him at his school, Martin Luther King High in Mount Airy, and charged him with witness intimidation and terroristic threats.

Anderson, of McMahon Street near Locust Avenue, East Germantown, was being held after a detention hearing yesterday at the Juvenile Justice Services Center in West Philadelphia.

Also yesterday, the District Attorney's Office announced that it would prosecute Anderson - who has two prior arrests for robbery and theft - as an adult.

"The actions of this teenager could have lasting repercussions on untold cases here in Philadelphia," District Attorney Seth 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 fullest extent of the law. I don't care how old you are: If you intimidate a witness in this city, I'm going to come after you."

Police hadn't determined yesterday how Anderson was connected to the anonymous Instagram account rats215, which revealed details about more than 30 witnesses of violent crime in Philly.

Figuring out how Anderson came across the photos of the court documents posted on his Twitter account last month could prove extremely difficult, said a law-enforcement source familiar with the case.

Some of those documents also were later posted to the rats215 Instagram account, which was shut down last week.

Authorities have not determined how Anderson got the documents, which were secret grand-jury evidence. Last year, in an attempt to counter a pervasive no-snitching culture in many neighborhoods, the D.A.'s office expanded the role of grand juries, in which proceedings are not public record and witnesses' privacy is supposed to be closely guarded.

Anderson's mother, Hope Anderson, insisted yesterday that her son is "a good kid" and innocent, and did not post the information maliciously. She also denied his connection to the Instagram account.

"He don't have nothing to do with that website," she said. "I can't even tell you who posted that stuff. My son can't even tell you who's posting that stuff."

She added, "He's not a kid that's bad and running around in the street. He's not trying to cause harm to nobody. These are . . . children just speaking about their opinion and them not realizing the effect sometimes of what they put on social media."

Sources say detectives began investigating Anderson's Twitter account Oct. 24, a week after he first posted court documents about a June 2012 drive-by shooting in Southwest Philadelphia. Posted on Anderson's feed were pictures of two victims in that case, witness statements, photo arrays and the suspect's affidavit.

The Police Department's Criminal Intelligence Unit continues to investigate the rats215 Instagram account.

In 2011, the teen's mother filed a federal complaint against the city and Philadelphia police, claiming that officers roughed up her son and falsely arrested him after suspecting him of a shooting in Southwest Philadelphia. A jury ruled against the Andersons and ordered them to pay $2,614.80. An appeal earlier this year was denied.

- Staff writers David Gambacorta and Morgan Zalot contributed to this report.

On Twitter: @DanaDiFilippo
Blog: phillyconfidential.com