ENGLEWOOD, Colo. -- Denver Broncos linebacker Brandon Marshall says it was "extremely concerning" to receive a letter filled with racial slurs and threatening language.

"Just so many racist, derogatory terms that were in it ... just a bunch of crazy things that was in it," Marshall said Friday. "That's what stood out to me, that somebody would have that much hate or malice toward me or put that much energy into writing something like that and send it to me. It takes so much energy to hate. I just don't understand it."

Marshall posted photos of the handwritten letter -- which includes multiple threats and racial epithets -- on his Instagram account Friday. The writer told Marshall "your time is coming, watch out," and "we are 'channeling' a devastating hard hit for you. Something to make you an invalid in a wheel chair."

The letter was sent with a return address of Mrs. Jackson's sixth-grade class at Martin Luther King Middle School, and "We love you, Mr. Marshall" was written on the envelope. Marshall called that a cover to make sure he received the letter.

"My first reaction was, I was taken aback, actually," Marshall said. "I had to read it over again. Did somebody really say this to me?'

"It said, 'You're a great player,' so I had a smile on my face, and then 'not.' And I'm like, whoa. I actually let Shane [Ray] read it after I read it. ... Disgusting, disheartening, deplorable -- you can use a bunch of words for it."

The Broncos' security staff is investigating the letter.

"I'm not afraid. I turned it over to team security, and we'll see what happens," Marshall said. "It's not going to spook me. It's not going to make me move any different than I've moved. It's definitely eye-opening."

Coach Gary Kubiak said after practice Friday that the "organization is on top of it."

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

He said he has received an outpouring of support on Instagram since making his post.

"I just wanted to show that ... to expose that racism still does exist,'' Marshall said. "A lot of people think, 'Oh, it's over; it's not out there,' but it really is.

"I wanted to expose that and that there are people like that and we still have a long way to go as people. I wanted to expose that people still hate each other ... whether it's because of your belief system or the color of your skin or just because I'm not like you, you're not like me."

Marshall took a knee during the national anthem before the Broncos' first eight games this season as a response to social injustice, including several shootings of black people by police nationwide. He met with Denver's police chief and has donated money for every tackle he has made this season to organizations that work with youth.

He said in November that he intended to stand for the anthem for the remainder of the season but would keep working for positive change.

The letter is the second known instance of an NFL player being targeted with racial abuse this week. On Tuesday, New York Giants fullback Nikita Whitlock reported a break-in during which burglars drew a swastika and "KKK," as well as the word "Trump," on walls inside his home.