Billy Joel has hired a bodyguard to protect his daughter Alexa Ray Joel after a stalker followed the young woman for months, even writing frightening messages on her Facebook page.

Alexa Ray Joel was allegedly so traumatized by the threatening messages that she was afraid to leave her Manhattan home.

Alexa's mother and Billy Joel's ex-wife, Supermodel Christie Brinkley, was also "terrified" that something would happen to her daughter. So, Joel hired a bodyguard to protect Alexa before reaching out to friend Paul McCartney to see if he knew the number of a trustworthy private investigator. McCartney allegedly "recommended a Europe-based private-security firm" that tracked Finley's login information on computers to Austin, Minnesota.

The private firm located the stalker naked in the woods in Minnesota in January, arresting the woman with the help of local police. The stalker, 40-year-old Sheryl Finley, was charged with stalking Alexa Joel.

Upon her arrest, Finley told local police that the threats she made against Alexa were not things she wanted to do "during this life."

Finley had reportedly spent months in a mental institution in 2010. She wrote more than 60 messages on Alexa Ray Joel's Facebook page. Some of the notes involved pedophilia, sadistic-sexual behavior, violent physical assaults and murder.

Finley is accused of using a fake Facebook account under the name "Rick Steenfield" to send Joel messages that focused on rape and murder.

The Daily Telegraph reported that Finley was charged with a felony for stalking Joel through messages sent by false impersonation and with a gross misdemeanor for stalking via repeated electronic messages.

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

Former FBI agent and current ABC news consultant Brad Garrett said that most celebrity stalkers are driven by delusion. He said, "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."

Finley pleaded not guilty to the charges, and she has a pre-trial hearing scheduled for May 3. 

The Joel family hasn't yet commented on the case.