Protesters march on the West Side Highway in lower Manhattan in New York City as thousands of demonstrators took to the streets demanding justice for the death of Eric Garner December 4, 2014. [Photo/Agencies]

Although chokeholds are banned by New York City police regulations, the 2,000-page patrol guide is vague about whether such use of force can be allowed under certain circumstances, said Maria Haberfeld, who heads the law and criminal justice department at John Jay College.

That gray area, she said, may have influenced the grand jury inquiry and could play a role in determining whether Pantaleo later faces departmental discipline.

PROTESTS SPREAD

The panel's decision sparked protests by hundreds of people who swarmed the streets of midtown Manhattan on Wednesday night, culminating in 83 arrests, according to police.

People also demonstrated in other cities, including Oakland, California, Washington, D.C., Denver and Minneapolis.

Demonstrators in New York City and elsewhere returned to the streets on Thursday evening. Hundreds of chanting protesters gathered for a noisy but peaceful rally in lower Manhattan, then began marching toward the Hudson River, snarling evening rush-hour traffic.

In Minneapolis, dozens of protesters blocked northbound traffic on Interstate 35W on Thursday afternoon, at times marching or lying down in the middle of the highway, escorted by police in squad cars seeking to keep the demonstrators moving.

The protests were largely peaceful, in sharp contrast to the night of arson, looting, vandalism and sporadic gunfire that swept St. Louis suburb of Ferguson, Missouri, after a grand jury decided on Nov. 24 not to indict Ferguson police officer Darren Wilson for fatally shooting 18-year-old Michael Brown.

The Justice Department has already launched a separate probe of the Aug. 9 shooting in Ferguson. Holder acknowledged on Thursday that the bar for bringing federal civil rights charges is high but said his department has met that standard when appropriate.

Michael Selmi, a professor at George Washington University and former litigator in the Justice Department's Civil Rights Division, said announcing a federal investigation following a controversial state grand jury decision is a tool the Justice Department can use to ease tensions, though it rarely leads to a conviction.

He pointed to the federal investigation into the case of George Zimmerman, the neighborhood watchman who shot and killed unarmed black teen Trayvon Martin in 2012, which has yet to be concluded.