Protesters in Baltimore last week. (Marvin Joseph/The Washington Post)

The ongoing conversation over how police officers use force during encounters with black men and boys has hopscotched across the country since last year, as new incidents have shifted the focal point from suburban St. Louis to New York City and from Cleveland to Madison.

As these protests have stretched from the end of one summer into the beginning of another, the public’s opinion has shifted, and a growing number of Americans now say these individual situations are part of a bigger problem.

In recent days, the hub for this conversation has been Baltimore, where large demonstrations and some rioting followed the death of Freddie Gray after he was fatally injured in police custody. The Justice Department announced Friday that it will investigate the Baltimore Police Department to see if it has engaged in discriminatory policing, making this the second federal investigation launched in the wake of Gray’s death.

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Gray’s death, like the deaths of Michael Brown, Eric Garner and Tamir Rice last year, sparked protests and unrest, with community outrage pouring out amid calls for larger reforms. These incidents and others became intertwined in the public consciousness and the ongoing debate, even as the precise circumstances of the deaths and the ensuing response, official and otherwise, have varied from place to place.

Still, an increasing number of Americans now think that the deaths of Brown, Garner and most recently Gray are part of a bigger problem involving the way police officers treat black people rather than isolated incidents. The biggest change in recent months? White Americans have started to change their minds about the issue.

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Public opinion on the topic is still split, with a little under half (49 percent) of Americans saying these deaths were part of a broader policing problem, according to a new poll from the Public Religion Research Institute. About four in 10 Americans say these were all isolated incidents.

But that’s essentially the inverse of how people felt just five months ago, when a little more than half of Americans (51 percent) said they thought these deaths were isolated incidents and four in 10 Americans said they were part of a broader problem. That is from a Washington Post-ABC News poll conducted in December, shortly after grand juries in Missouri and New York decided not to indict police officers over the deaths of Brown and Garner, respectively.

The difference since December has come from white Americans, who are less likely to view these deaths as isolated incidents now. The new survey shows that 45 percent of white Americans said the deaths were isolated, down from 60 percent in The Post’s poll last December; the number of white Americans who say there is a broader problem in policing has moved to 43 percent from 35 percent over the same span.

The months since that earlier poll have seen deaths and protests in places like North Charleston, S.C., where a white officer was arrested and charged with murder after video showed him firing multiple shots into the back of a fleeing driver, and Baltimore, where six police officers were charged for Gray’s death.

These incidents can have a cumulative effect, something experts say has been clear in the way authorities in these places moved very quickly to announce charges and try to quell unrest to avoid, as has been said more than once, “another Ferguson.” The end result is that incidents that would have been local news stories a year ago “now [appear] to fit this larger national narrative,” David A. Harris, a law professor at the University of Pittsburgh and an expert in police use of force, told us last week.

Of course, this shift in opinion still leaves huge gaps in the ways different people view the issue. For one thing, white and black Americans really see things differently. About three out of four black Americans say there is a broader policing problem, which is 30 percentage points higher than white Americans. (The opinion among black Americans surveyed did not change since December.)

There are also big differences based on political preferences, something that will likely continue to inform how politicians (including the array of people stampeding toward the presidential election next year) react to new incidents, new protests and new calls for reform.

“Democrats and Republicans understand the deaths of Michael Brown, Eric Garner and Freddie Gray in dramatically different ways,” Daniel Cox, the Public Religion Research Institute’s research director, said in a statement accompanying the poll results.“Where Democrats see a pattern, Republicans see a series of isolated incidents. These deep partisan divisions will almost certainly be reflected in the way each party assesses issues within the criminal justice system.”

Democrats are much more likely to think there are bigger policing problems; six-in-10 Democrats feel this way, double the number who think the incidents are isolated. About two out of three Republicans believe they are isolated incidents, nearly three times the number of Republicans who see broader policing problems. People who say they are independent tend to align more with the national averages.

And there are still huge divides in how people view the criminal justice system overall, the Public Religion Research Institute poll shows. White Americans are pretty split on whether or not people of all races are treated equally: 47 percent don’t think so, just ahead of the 46 percent who do think the system treats everyone fairly. About half of Hispanic Americans think the system doesn’t treat everyone fairly. Meanwhile, about four out of five black Americans say say white people and non-white people do not receive the same treatment.

Related: The generational gap in how the Baltimore protests were viewed