It is rare for either federal or state prosecutors to charge a US police officer for excessive force, even when a death results.

The US Supreme Court and lower courts have ruled over decades that police officers should have wide latitude to use violence to defend themselves and to take suspects into custody.

"There are a lot of cases where police officers don't get indicted for what looks like extreme situations," said Aaron Mysliwiec, president of the New York State Association of Criminal Defense Lawyers. "Many jurors and judges tend to believe police officers more than your average witness."

In ruling Garner's death a homicide, the city medical examiner said police officers killed him by compressing his neck and chest. His health problems, including asthma and obesity, were contributing factors, the medical examiner said.

The video of Garner's arrest shows him arguing with police officers, saying, "Please leave me alone," and later, "Don't touch me," before a group of four officers tackled him to the ground. He then began to plead with them, saying repeatedly, "I can't breathe."

Shortly before the grand jury announcement, the New York City police launched a pilot program to equip officers with body cameras. The video camera program was ordered by a federal court judge who ruled last year that police had unfairly stopped and frisked black and Latino New Yorkers. It aims to make officers more careful and accountable about using force, de Blasio told reporters, while reducing complaints and lawsuits.