“It’s hard to overcome an officer’s subjective belief that he needed to use deadly force,” said Jeffrey Urdangen, a longtime Illinois defense lawyer and the director of the Center for Criminal Defense at Northwestern University’s law school.

Image Laquan McDonald Credit... via Associated Press

Still, some officers are convicted of murder, including last month in Texas, and the video of Laquan’s death could give prosecutors an advantage. Officer Van Dyke’s lawyers would have to convince jurors that Laquan was the aggressor that night.

“That’s a tall order for the defense, in my view,” Mr. Urdangen said.

Only one black person was chosen for the jury

Race is central to this case. During jury selection last week, each side accused the other of excluding people based solely on skin color. Prosecutors said defense lawyers unfairly removed black people from the jury pool; Officer Van Dyke’s lawyers said the prosecution wrongly excluded white men.

In the end, of 12 Chicago-area residents chosen for the jury, six appeared to be white, three Hispanic, one black and one Asian. The twelfth juror appeared to be white or Hispanic. (Almost one-quarter of people in Cook County, which includes Chicago, are black, and about 40 percent are white.)

It had been unclear until Friday whether a jury would even hear the case. Officer Van Dyke’s defense lawyers tried and repeatedly failed to get the trial moved out of Cook County, where police-community relations are strained, to a suburban or downstate county with a higher proportion of white people and more reverence for law enforcement.

“There’s people out there, especially in Chicago,” said Alan Tuerkheimer, a jury consultant, “who think there’s this code of silence, that police officers look out for each other, that they use excessive force.”

Police officers charged with crimes often prefer that judges — not juries — hear their cases, but Officer Van Dyke’s lawyers ultimately chose to have a jury. They had the option of putting the case before a judge, but that decision would have put the verdict in the hands of Judge Vincent Gaughan. He is viewed as a mercurial jurist, having briefly jailed Officer Van Dyke earlier this month and repeatedly threatening to hold Daniel Herbert, the lead defense lawyer, in contempt.