The shaky and frenetic video, lasting less than a minute, appears to show two New York police officers holding a man on the floor, with one repeatedly slamming his right fist into the man’s face.

The man, Luis Solivan, 19, was later charged with assaulting an officer, but his case was dismissed after a grand jury watched the video, which an acquaintance shot through an apartment window in the Bronx, his lawyers say. Now, that same footage may emerge as crucial evidence in a civil rights lawsuit that Mr. Solivan’s lawyers filed on Monday in Federal District Court in Manhattan.

The lawsuit charges that the officers, after chasing Mr. Solivan into his family’s apartment on University Avenue on Nov. 14, engaged in a “brutal and sadistic” beating beyond what the video captured, also using pepper spray and slamming his head against a wall after he was handcuffed.

The officers have provided a different account. In a criminal complaint against Mr. Solivan, they charged that he attacked both officers and tried to take one of their guns. But a Bronx grand jury declined to indict Mr. Solivan on any of the charges.