Fellow officer writes moving tribute to Philadelphia Police Ofc. Hartnett

PHILADELPHIA (WPVI) -- An incredibly moving post from one of Ofc. Jesse Hartnett's brothers in blue - a now viral Facebook letter to a hero.

In the letter, Ofc. Jonny Castro recounts the assault moment by moment, starting with the fact that Hartnett was working alone last night when the gunman fired shots into his vehicle.

Castro writes: "You ducked your head down at just the right moment that rounds actually missed your head by inches. You drew your service weapon and viciously returned fire. It was literally a fight for life and death."

Castro goes on to say: "I was only 2 minutes away, but it felt like it took me an hour. We almost crashed 3 separate times trying to get to you."

Read the full letter below: