Even though the takedown of Mr. Garner, 43, was preserved on a cellphone video, prosecutors will have to prove that the amount of force used by Officer Daniel Pantaleo was unreasonable or excessive, and that the officer acted willfully to deprive Mr. Garner of his civil rights, Mark F. Pomerantz, a former federal prosecutor, said.

“It’s crystal clear that the federally protected right that we’re talking about is the right to be free of an unreasonable seizure of his person — that appears in the Bill of Rights,” said Mr. Pomerantz, who helped win the 1998 civil rights conviction of a former police officer, Francis X. Livoti, in the chokehold death of Anthony Baez in the Bronx.

Mr. Pomerantz added, however, that there was “no precise definition of when force is reasonable or not.”…

In the Garner case, a key question will be whether the government can show willfulness on the part of Officer Pantaleo — the high standard of intent that the Supreme Court has required, said Daniel C. Richman, a former federal prosecutor who teaches criminal law at Columbia.