A protestor marches with a sign after a gathering to show solidarity with the family of black teenager Michael Brown outside the American Embassy in London November 26, 2014. [Photo/Agencies]

The Justice Department has been investigating whether to bring federal civil rights charges against Wilson and the police department.

"The sad fact is that it brings up issues that we've been struggling with in this country for a long, long time," said Matthew Green, an associate professor of politics at the Catholic University of America.

"These are not problems and issues that are going to get resolved by one president in the remainder of his term."

Wilson told ABC News that there was nothing he could have done differently that would have prevented Brown's death. But the parents of the slain teenager said they did not accept the officer's version of the events.

"I don't believe a word of it," Brown's mother Lesley McSpadden told "CBS This Morning" on Wednesday.

The crowds in Ferguson were smaller and more controlled than on Monday, when about a dozen businesses were torched and others were looted amid rock-throwing and sporadic gunfire from protesters and volleys of tear gas fired by police.

Two FBI agents were shot and wounded at a house in north St. Louis County early on Wednesday and the suspect was dead in an incident not related to the racially charged unrest around nearby Ferguson, officials said.

Protests over the Ferguson decision in several major US cities shut highways on Tuesday night and led to hundreds of arrests. In New York on Wednesday, there were more police than the dozen or so protesters at Union Square, a gathering point for Tuesday's rally in the city.