The death of a young black man in police hands, just like in Ferguson, Mo., New York City and North Charleston, S.C., triggered unrest in Baltimore.
"Riots invariably begin with some form of police action that is interpreted as a form of brutality," said Michael Flamm, a historian at Ohio Wesleyan University who is working on a book about urban riots. "But then the riots reveal wider and deeper problems that bring people into the streets."
President Obama alluded to this dynamic on Tuesday. While saying "there's no excuse" for violence, he noted that many communities "have been stripped away of opportunity" and thus are scenes of ongoing despair.
"It was symptomatic of frustration that is boiling up -- lack of jobs, lack of education, and people are frustrated with the police," said Deborah Scott, executive director of Georgia Stand-Up, an Atlanta nonprofit that provides job training and other services to low-income residents. "It's an opportunity to vent about the injustices that have happened for years and years."
The West Baltimore neighborhood where Freddie Gray lived, Sandtown-Winchester, suffers all the ills associated with what used to be called the urban underclass. Domestic violence, truancy, high rates of incarceration and poverty are all big and persistent problems.
Sandtown's murder rate is double that of Baltimore as a whole, while the share of its population that fails to finish high school is about three times as high. Houses and other buildings are about four times as likely to be abandoned in the neighborhood than the city's averagee.
The revitalization efforts that have spruced up parts of Baltimore and many other center cities in recent years have passed such communities by. Even cities that are doing well have pockets of poverty that are not sharing in the prosperity.
"The investment that has come has not included African Americans," said Mindy Fullilove, a Columbia University psychiatrist who studies urban displacement. "Their situation, especially for the poorest, is intolerable."
We've been here before. Back in 1968, the report of the Kerner Commission -- a presidential commission headed by then-Illinois Gov. Otto Kerner -- concluded the riots of the period were rooted in black frustration at lack of economic opportunity. It became a bestseller.
But while liberals witness the unrest in Baltimore and come away impressed with the need to address or at least discuss longstanding urban ills, conservatives respond that most residents are law-abiding and don't throw bricks or start fires. They blame figures such as Baltimore Mayor Stephanie Rawlings-Blake for not doing more to protect property. Rather than looking to government to offer solutions, some conservatives are blaming dependence upon government for causing the problems in the first place.
In a blog post, City Journal Associate Editor Matthew Hennessy suggested that liberal solutions have failed to make conditions better in West Baltimore. Sandtown itself was the site of a $130 million community building initiative back in the 1990s.
"In so much as anyone in Baltimore is angry at the 'system,' they should direct their rage where it belongs -- an unbroken, five-decade string of one-party [Democratic] rule in the city and a national war on poverty that has systematically dismantled the black family," Hennessy wrote.
Whether you describe urban unrest as rioting or rebellion, it's clear we're in a period when such actions are spreading. On Tuesday night, while Baltimore was mostly quiet thanks to curfew, renewed protests occurred in Ferguson and Chicago. Two people were shot in Ferguson.
Given the events of recent months, the potential is certainly there for protests and possibly violence to break out in many other areas around the country. As a result, mayors and police chiefs are asking themselves how they can address frustration in their cities, perhaps considering proactively reaching out to wide cross-sections of their communities so that some level of trust might exist if trouble comes.
"I don't think any mayor of any city of any size can rest assured that they'll have no trouble this summer," Flamm said.