Two other officers who may have been involved in the incident have not been identified, authorities said. The hit-and-run driver, meanwhile, remains at-large.

Prosecutors and forensic investigators said they've determined that Chinedu died of injuries suffered after he was struck by a vehicle on Calzada de Tlalpan, but said his body showed trauma from blows delivered by at least two auxiliary police officers, whose actions were captured by surveillance video. Four officers have been questioned in the incident, but there have been no arrests or charges filed.

"This was an act of discrimination," Castillo told a newscast. "Why? Because they detained him simply because he was black. He wasn't doing anything. Isaac was waiting for a taxi to return to his home in peace."

The case of Isaac Chinedu has led to demonstrations among Mexico City's African and Afro-Mexican communities, which are laying blame on the police officers who allegedly beat the 29-year-old before he apparently ran into traffic on a busy highway. Chinedu's Mexican widow, Liduvina Castillo , claims that racial prejudice resulted in her husband's death, a charge activists here are rallying around.

Shortly after midnight on May 11 in central Mexico City, Isaac Chinedu , an immigrant from Nigeria, became involved in some kind of confrontation with a group of police officers on a dark side street. The encounter escalated, and Chinedu was severely beaten. Some minutes later, he was dead, the victim of a hit-and-run driver, authorities say.

Supporters have held two protests this month seeking justice for Chinedu and bringing attention to a small but growing community of black foreigners in Mexico (links in Spanish). Hundreds of refugees arrived from Haiti after the January 2010 earthquake, as La Plaza has reported. Chinedu had come to Mexico as a refugee from Nigeria, reports said.

A civil organization, the Citizen's Committee in Defense of the Naturalized and Afro-Mexicans, staged the demonstrations and plans to initiate a fast Thursday morning on the doorstep of City Hall. The group's president, Wilner Metelus, a native of Haiti and long-term resident of Mexico, said group members seek a meeting with Mayor Marcelo Ebrard about the death of Chinedu.

"We no longer believe what the attorney general says about the case. There are many contradictions," Metelus said in an interview with La Plaza. "We want a direct dialogue with Marcelo Ebrard on the matter. How could any human being receive such treatment?"

Chinedu had lived in Mexico City for about 10 years. He was a legal resident and the father of two Mexican-born children. As the incident was reported by the local press, revelations surfaced that Chinedu had spent two years behind bars on minor narcotics infractions, which were later dismissed. (In Mexico's deeply troubled justice system, such long prison terms for minor or even nonexistent offenses are quite common.)

On May 11, his widow says, Chinedu was leaving a Mother's Day party and waiting for a cab. The grainy video shows Chinedu rushing toward a passing police cruiser to flag it down. He then appears to try to force his way inside. Standing police officers drag and then violently yank Chinedu back to the sidewalk, where he is struck repeatedly, the video shows.

Neighbors came to Chinedu's aid and the officers left the scene, officials said. "What I did was hug him and separate him from the police so they would stop beating him," a witness told ForoTV.

A top official in the Mexico City attorney general's office said Sunday in a news conference that as paramedics arrived and began tending to Chinedu, he regained consciousness, kicking and struggling. Without explanation, he got up, ran into traffic, and was hit by a fast-moving passenger car.

Toxicology tests showed Chinedu was not under the influence or alcohol or drugs on the night he died, officials said. The investigation is ongoing.

-- Daniel Hernandez in Mexico City

Image: Screen-grab showing surveillance video of the incident in which Isaac Chinedu died May 11, 2011. Credit: ForoTV