Colin Kaepernick's national anthem protest seems to be slowly be gaining steam across the NFL.

During the NFL opener on Thursday, Broncos linebacker Brandon Marshall joined the cause by kneeling during the national anthem.

The Broncos linebacker was the only player from either team who kneeled before the game. Also of note: Marshall was a teammate of Kaepernick's in college when both played at Nevada.

The Broncos issued a statement after Marshall took a knee.

"While we encourage members of our organization to stand during the National Anthem, we understand and respect it being a personal decision," the Broncos said.

Marshall is the third player in two weeks to join Kaepernick's cause. Seahawks defensive back Jeremy Lane and 49ers safety Eric Reid both kneeled during national anthem before they played in their final preseason game.

After the game, Marshall clarified this reasoning for kneeling during the anthem.

"I'm not against the military. I'm not against police or America. I'm just against social injustice," Marshall told reporters after the game, per to the Denver Post. "I prayed about this long and hard and decided this is what I wanted to do."

Marshall said he didn't tell any other Broncos about his decision, other than tight end Virgil Green, and said he plans to keep kneeling going forward. He said he also plans to donate an undisclosed amount of money to causes benefiting veterans.

"I looked at my phone and I already have a bunch of hate comments and this is why I'm doing this," Marshall said.

When the rest of the NFL schedule kicks off this weekend, Marshall, Kaepernick, Lane and Reid could have some company in the protesting department. The Seahawks have hinted they plan on protesting as a team during their season opener against the Dolphins on Sunday.

Kaepernick has made it clear that he's not going to stand for the national anthem until police brutality against minorities comes to an end.

"There's a lot of things that need to change," Kaepernick said on Aug. 28. "One specifically? Police brutality. There's people being murdered unjustly and not being held accountable. People are being given paid leave for killing people. That's not right. That's not right by anyone's standards."