Evan Vucci/Associated Press
According to Charles Robinson of Yahoo Sports, Kaepernick's team is attempting to show any potential influence the Trump administration had on Kaepernick's free agency. CNN's Jill Martin, citing a source familiar with the ongoing grievance process, later confirmed Robinson's report in an email provided to Bleacher Report.
Kaepernick contends teams colluded to not sign him due to his protests against racial inequality and police brutality during the national anthem in the 2016 season.
Per Robinson, Kaepernick's team has learned that some NFL owners had direct conversations with Trump regarding player protests during the anthem. The purpose of subpoenaing Trump and Pence is to find if the Trump administration applied any pressure to NFL teams to avoid Kaepernick.
Trump and Pence have both been vocal opponents of protesting during the anthem.
Last year at a political rally in Alabama, Trump said: "Wouldn't you love to see one of these NFL owners, when somebody disrespects our flag, to say, 'Get that son of a b—h off the field right now. Out. He's fired! He's fired!'"
In October, Pence attended a game between the Indianapolis Colts and San Francisco 49ers, but he tweeted he left early because he would "not dignify any event that disrespects our soldiers, our Flag, or our National Anthem."
Kaepernick was no longer with the Niners last season, but former teammate safety Eric Reid was among the players who protested.
Reid is currently a free agent and has filed a collusion grievance against the NFL.
Per Robinson, Kaepernick's legal team must convince an arbitrator that testimony from Trump and Pence would be relevant in order to subpoena them since the president and vice president stretch "beyond the boundaries of the agreement between the NFL and the player's union."