Wichita ? For 31 years, it remained one of the most closely guarded secrets in the BTK serial murder case.

Wichita police used it to help verify that communications sent to the police and media actually came from the killer. They kept it secret so it wouldn’t be used in hoaxes.

It was the serial killer’s signature — a sexually suggestive configuration of the letters “B,” “T” and “K” still etched into the memories of some former detectives.

Instead of writing the letters in a line from left to right, the killer stacked the “B,” “T” and “K” from top to bottom, with the B shaped to look like a woman’s breasts.

Last week, prosecutors charged Dennis Rader with 10 counts of first-degree murder in killings tied to BTK from 1974 to 1991 in and around Wichita. The former Park City compliance officer remains in the Sedgwick County Jail.

Tony Ruark, a Wichita psychologist who consulted with police on the BTK case from 1979 to 1981, recalls that he was told to keep the unique signature a secret.

“This is the one thing that couldn’t get out,” he said.

The signature appeared in two of five known communications from the killer from 1974 to 1979, according to research by Robert Beattie, the Wichita man who is writing a book about the case.

The signature also showed up on a letter that arrived in March 2004 at the Wichita Eagle.

At the request of Wichita police, The Eagle agreed at the time not to describe the signature.