Racially charged threats mailed to Denver Broncos LB Brandon Marshall

Denver Broncos linebacker Brandon Marshall was the recipient of a letter filled with racial slurs and threatening language, likely prompted by his recently concluded stance of not standing for the national anthem.

Marshall posted photos of the letter on his Instagram account on Friday and said it represents an example of why he took a stance against social injustice. He said he received the letter two weeks ago.

The letter is filled with racist remarks and contains the threat, "your time is coming, watch out (expletive!)"

Marshall responded to the contents of the letter on the Instagram account.

"The hatred by some against people of color is one of the reasons we are where we're at in the world today," Marshall wrote, "and they wonder why we feel the way we do and take the stances that we take. I received this letter at work."

The Broncos' security staff is investigating the letter.

Marshall began publicly taking a stand shortly after San Francisco 49ers quarterback Colin Kaepernick took a knee while the anthem was playing in a preseason game in August.

Marshall and Kaepernick were college teammates at Nevada.

At the time, Marshall expressed the reasoning for the anthem boycott this way: "I'm not against the military, I'm not against the police, or America. I'm against social injustice."

Marshall began standing for the anthem again in November but said he will keep working for change. He is donating money to youth groups for every tackle he makes.