Gregory Pace / BEImages / Rex USA 1 / 5
Gregory Pace / BEImages / Rex USA 1 / 5

Singer Billy Joel hired Sir Paul McCartney's private investigator to track down a woman charged with stalking his daughter.

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Alexa Ray Joel allegedly received over 60 threatening Facebook messages from Sheryl Finley over the past few months, prompting her father to call on chiefs at a European security firm, who previously worked for his pal McCartney, to launch an investigation into the case.

A source tells the New York Post, "Her mother, Christie Brinkley, was terrified that someone would harm her baby. Billy Joel was worried but focused."

Detectives ultimately found 40-year-old Finley, who spent time in a mental institution in 2010, in Austin, Minn., and she has since been charged with felony and gross misdemeanor stalking for repeatedly sending electronic messages.

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