HARTFORD, Conn. (AP) — Skull and Bones, the secret society at Yale University, is warning of an impostor who has called some students purporting to recruit them and then asking them to complete a humiliating challenge.

The campus society has figured prominently in books, films and conspiracy theories. Its secrecy has fueled the public's curiosity about a group that counts former Secretary of State John Kerry and both Presidents Bush among its past members.

The note sent out to members of Yale's junior class through the student government was a rare public statement, according to David Alan Richards, author of "Skulls and Keys: The Hidden History of Yale's Secret Societies."

"While famously its mantra has been 'Never respond, never explain,' because it doesn't see itself as a public organization, in today's climate to have allowed that to happen could conceivably damage the society's reputation," Richards said. "It was a foul thing to do."

The Yale police department has received three complaints of harassing phone calls from somebody claiming to be from Skull and Bones, according to university spokesman Tom Conroy, who said the cases remain open. Some students described an anonymous caller who instructed them to hand their phones to somebody nearby and that asked that person questions about the student's sex life.

Cole Addonizio, a Yale junior, said he suspected it was a prank soon after he received a call from somebody who said it was the start of the "tap" process. He played along and handed his phone to his brother, who was asked inappropriate questions. They hung up on the caller.

"I figured it was a prank since I hadn't heard about them calling them like this," Addonizio said.

Skull and Bones, which dates back to 1832, said in its note to students that the prank caller was exploiting the society's "mysterious nature" and encouraged people who received such calls to report the incident to Yale police or their college dean.