The group’s plan emerged on Sunday when James, the star of the Cavaliers, told reporters that Rose’s T-shirt was “spectacular” and that he was looking for one of his own. Organizers at Justice League NYC had a conference call that night.

Rose’s public protest had resonated with members of the group, as did James’s support of it. And with the Nets about to host a fairly large event on Monday — their game against the Cavaliers would be attended by the royal couple, Prince William and Catherine, Duchess of Cambridge, and more than 100 credentialed members of the news media — they saw a platform to broadcast their message about justice reform, members of the group said.

The plan was simple, really: Deliver replicas of the “I Can’t Breathe” T-shirt to James and other players who might be interested in wearing them before the game. Executing that plan, though, would be far more complex, as intermediaries between Jay-Z and James would need to get involved, none of the T-shirts actually existed yet, and the game was less than 24 hours away. But those who were aware of the plan were confident.

“We knew we were going to make it happen,” said Michael Skolnik, Simmons’s political director.

That same night, after the conference call, Rameen Aminzadeh, a member of Justice League NYC, drafted a simple design for the text of the T-shirt, which other members of the group approved sometime after 1 a.m. The plan was in motion.