(CNN) Boston Marathon bombing survivor Rebekah Gregory, who lost her leg in the April 15, 2013, attack, saw her attacker in person for the first time Wednesday, on the first day of Dzhokhar Tsarnaev's trial.

Gregory walked to the witness stand, thanks to a prosthesis, and gave some of the day's most emotional testimony.

Afterward, she wrote a public letter to Tsarnaev, which she posted on her Facebook page

JUST WATCHED New video of Boston Marathon bombing shown to jurors Replay More Videos ... MUST WATCH New video of Boston Marathon bombing shown to jurors 02:31

Dear Dzhokhar Tsarnaev,

My name is Rebekah Gregory. We don't really know each other and never will. But over the last two years, I have seen your face not only in pictures, but in almost every one of my nightmares. Moments before the first blast, your stupid backpack even brushed up against my arm, but I doubt you remember because I am no one to you. A complete stranger. And although I was merely just a blip on your radar, (someone that happened to be standing 3 feet from your designated "good spot" for a bomb), you have been so much more to me. Because you have undoubtedly been my source of fear since April 15th, 2013. (After all, you are one of the men responsible for nearly taking my child, and for the permanent image embedded in my brain of watching someone die.) Up until now, I have been truly scared of you and because of this, fearful of everything else people might be capable of.

Read More