One source said Kaepernick is "in the best shape of his life."
The 32-year-old last played for the Niners in 2016. He opted out of his contract that offseason, with general manager John Lynch confirming the team had planned to release him.
Many believe NFL owners effectively blackballed Kaepernick in response to his social activism. He began protesting during the national anthem during the 2016 season, telling NFL Network's Steve Wyche he was "not going to stand up to show pride in a flag for a country that oppresses black people and people of color."
Last November, he staged a workout for any interested suitors, which was largely overshadowed by logistical issues in the days leading up to the event. Some questioned whether it was truly a good-faith effort by the NFL to get Kaepernick back in the league.
To some extent, recent events have vindicated Kaepernick.
A video showed former Minneapolis police officer Derek Chauvin kneeling on Floyd's neck for nearly nine minutes while Floyd was in custody. Floyd told officers at the scene he couldn't breathe, and he died at a local hospital.
Chauvin is facing charges of second-degree murder, third-degree murder and second-degree manslaughter, and three officers who were at the scene were charged with aiding and abetting murder.
NFL Commissioner Roger Goodell issued a statement last Friday saying the league erred in not supporting Kaepernick and other players who were outspoken about social issues:
Numerous Americans have marched and protested to demand reforms addressing police brutality and systemic racism in the wake of George Floyd's killing.