Pop legend Billy Joel has reportedly hired a bodyguard to protect his daughter from a woman who allegedly stalked her for months and wrote her frightening messages on Facebook.

Billy Joel, whose many hit songs include "Just the Way You Are," hired a private security firm to find the woman, the New York Post reported this weekend. The security firm was reportedly recommended to Joel by Beatles superstar Paul McCartney.

The alleged stalker, 40-year-old Sheryl Finley, was found naked in the woods in Minnesota in January, the Post also reported, adding that Finley was charged with stalking Alexa Ray Joel, 27, the daughter of Joel and his ex-wife, supermodel Christie Brinkley.

Finley, who had reportedly spent months in a mental institution in 2010, reportedly wrote more than 60 messages through Facebook, some of which involved pedophilia, sadistic-sexual behavior, violent physical assaults and murder, the Post said, citing court records.

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Alexa Ray Joel, who is also a song writer, was reportedly afraid to leave her Manhattan home.

Finley, of Austin, Texas, is accused of using a fake Facebook account under the name "Rick Steenfield" to send messages to Joel that focused on rape and murder. The arrest happened in January.

"We need to take the harassment and the threats that occur, via those digital means, very seriously," Austin Police Department Capt. Dave McKichan told ABC News' TV affiliate KAAL-TV in Texas .

According to The Daily Telegraph, a British newspaper, Finley was charged with a felony for stalking Joel through electronic messages sent by false impersonation, and with a gross misdemeanor for stalking via repeated electronic messages.

She had pleaded not guilty to the charges, and has a pre-trial hearing coming up in a few weeks.

Brad Garrett, ABC News consultant and former FBI agent, said many celebrity stalkers are driven by delusion.

"Delusion that they either have a relationship with the famous person. Or they desire one … You have to separate the vile, violent words to the ability of the person or even the motivation of the person to actually carry them out," he said.

The Joel family is not commenting on the case.