(CNN) Colin Kaepernick's legal team is expected to seek subpoenas for President Donald Trump, Vice President Mike Pence and other campaign officials relating to the quarterback's collusion case against the NFL, a source familiar with the ongoing grievance process has told CNN.

The new move intends to explore NFL owners' ties with Trump, the source said, adding that several owners, as well as NFL Commissioner Roger Goodell, have already been deposed in the case.

Brian McCarthy, a spokesman for the NFL, said the league had no comment. The expected move was earlier reported by Yahoo! Sports

Kaepernick ignited a controversy back in 2016 when he refused to stand during the National Anthem, protesting what he believes are racial injustices and ongoing police brutality in the US. He opted out of his contract with the 49ers last year and became a free agent, but no NFL team has signed him. He filed a grievance against the NFL in October, claiming teams are colluding to deny him a job.

The anthem controversy, meanwhile, has ignited a racially sensitive national debate over free speech, pitting mostly black athletes against white NFL owners, and Trump has seized on the issue to energize his political base

At a rally in Louisville, Kentucky, last year, Trump switched gears from talking about the nation's urban communities to calling out the quarterback.

"It was reported that NFL owners don't want to pick him up because they don't want to get a nasty tweet from Donald Trump," he said. "Do you believe that?"

Trump also has called out the NFL multiple times over the anthem controversy. He canceled the traditional visit by the Super Bowl champion Philadelphia Eagles earlier this week, initially claiming that the decision was due to players not standing for the anthem, though the White House later acknowledged that low expected attendance by team members was a key factor.

Pence left an October NFL game between the 49ers and Colts in Indianapolis after some players knelt during the National Anthem, saying on Twitter at the time that he would not "dignify any event that disrespects our soldiers, our Flag, or our National Anthem."

Last month, NFL team owners decided that players must stand during the National Anthem this season, a reaction to fierce backlash against some who took a knee in symbolic opposition to what they view as the systemic oppression of people of color, including by police.

Goodell said teams whose players and personnel do not stand and show respect for the flag and the anthem will be fined by the league. Each team, however, may develop its own work rules for those who don't comply with the policy, which allows players the option of remaining in the locker room during the playing of the anthem if they do not wish to comply.