PAULA PACE worked as a beloved teacher’s aide for nearly 15 years – until she was unmasked as an unspeakable monster.

School officials at BCLUW High School in Conrad, Iowa, never for a moment suspected that Paula was part of a blood­thirsty Indianapolis family who tortured and killed a helpless foster child back in 1965.

Although she’d been convicted of second-degree murder in the terrifying crime, background checks – if there were any – never detected it.

Incredibly, the gruesome kill­ing had not only gotten major news coverage but also spawned fiction and nonfiction books, a play and two movies – “An American Crime” and “The Girl Next Door.” But still no one in the school district seemed to have the slightest suspicion that the brute roamed freely among their precious children.

Just recently, police re­ceived an anonymous phone tip that exposed 64-year-old Pace as Paula Baniszewski, a key participant in the grisly torture and slaughter of 16-year-old Sylvia Likens. The caller had apparently found out about Pace’s horrifying past life from a Facebook posting.

“They wanted to make us aware of it because of the crime that was involved and because she was in the school system,” said Brundy County Sheriff Rick Penning.

The well-liked teacher’s aide had completed her prison sen­tence, was released on parole and had avoided trouble.

But when the bombshell dis­closure hit, she was fired from her school job – unable to escape her legacy as a participant in one of the most sickening crimes ever to shake Indianapolis.

It all began in the summer of 1965, when the parents of Sylvia and her sister Jenny left them in temporary foster care with Gertrude Baniszewski and her seven children.

After Gertrude’s daughter, Paula, and Sylvia had a falling-out, family members tortured Sylvia with burning cigarettes, beat her, branded her with a red-hot needle and inflicted other abuse, including starving her. They tattooed “prostitute” on her chest, tied her to her bed and would not let her use the restroom.

On Oct. 26, 1965, after months of abuse, the 16-year-old girl’s emaciated body was discovered in the basement of the demented family’s home.

Accounts from the time reveal that Gertrude and Paula were involved in the atrocity, along with Paula’s other siblings and even neighborhood children.

In May 1966, Gertrude was convicted of first-degree murder and Paula of second-degree murder. Both were thrown be­hind bars for life.

But in 1971, they were granted new trials on a technicality. Ger­trude was again slammed with a life sentence – and died of cancer in 1990. To avoid retrial, Paula pleaded guilty to a lesser charge of voluntary manslaughter and was released from prison in late 1972.

She changed her name and quietly moved to Iowa, where she began working for the school district in 1998.

To this day, the Internet is still filled with accounts of the heinous crime, but the biggest mystery remains: How did Paula Pace, aka Baniszewski, “hide” in plain sight?