The Typewriter

Glenn Close's character gets anonymous notes in the mail with important clues.

She sends the notes to some lab, and they can identify the typewriter model--through the ink and the fonts (right word?) it produces. The typewriter has a flaw, the lower-case "t" is slightly raised whenever it's typed.

After having victory sex with Jeff Bridges' character, she finds the typewriter in the room. She types a note--cue scary music--and it's the one that produced the raised t.

The sender gave her information that she couldn't find on her own and that wasn't publicly known (i.e. the attack on Julie Jensen 18 months before).

Since it's Jeff Bridges typewriter, he is the one who sent the notes, and she concludes that he did both attacks--the first to set up a red herring, the Bobby Slade character.

It was the writer's (Joe Eszterhas) faithful old typewriter, which subsequently got lost on its way back to him after its starring role. No-one knows who stole it.