Hayden Panettiere Was Scared of 'Dangerous' Ex-Friends During Book Release

Hayden Panettiere was afraid she'd get blowback from "dangerous" former friends when she released her memoir ... TMZ has learned.

Sources close to Hayden tell TMZ ... she'd told people close to her she was nervous about publicly releasing details of her tortured past.

We're told the actress was specifically concerned about publicizing the time a friend allegedly tried to set up a sex session with a naked male singer on a yacht when she was only 18.

Our sources say Hayden told her circle she felt she legally couldn't name her ex-pals in her book, and she considered these former friends to be "dangerous."

We're told Hayden's circle encouraged her to name names for accountability, but she was scared of the repercussions.

As you know, Hayden identified the woman who essentially delivered her to the naked man as Stella McAmis -- a fake name -- and didn't name the British singer from the traumatic yacht incident.

Sources tell TMZ ... Hayden told friends it was torture revisiting the painful parts of her life for the book. They say there were certain terrible experiences she left out involving famous people in Hollywood ... because she felt they were too disturbing to include.

As you know, Hayden tragically passed away Sunday after a call came in for someone in cardiac arrest at an Airbnb in Greenville, South Carolina. First responders attempted life-saving measures, but couldn't save her.

Her boyfriend Brian Hickerson and his brother, Zach, were both at the scene. Brian reportedly became visibly emotional when she was declared dead and showed police a bag of medication she was taking.