BALTIMORE » For nearly two years, ever since her brother Tyrone West died after a struggle with the police, a 35-year-old preschool teacher named Tawanda Jones has been in the streets here on Wednesday nights, protesting. Her message: "We need killer cops in cellblocks."

Though the officers involved in West’s July 2013 death have been cleared of wrongdoing, his case and other police-involved killings here are woven into Baltimore’s psyche, part of what Mayor Stephanie Rawlings-Blake calls the "broken relationship" between residents of this majority black city and a police department with a history of aggressive, sometimes brutal behavior.

That history helps explain the long-simmering anger that boiled over this week with the death on Sunday of Freddie Gray, 25, who suffered a severe spinal cord injury in police custody. Despite efforts by city officials to improve relations — Rawlings-Blake, alarmed by wrongful-death lawsuits, last year asked for a Justice Department review — thousands have staged protests that are expected to continue through the weekend.

The tensions date at least to 1980, when the NAACP called for a federal investigation into police brutality, and continued into the past decade with a crime-fighting strategy known as "zero-tolerance policing" that led to mass arrests. Since 2010, just one Baltimore police officer has been prosecuted for killing a civilian: an off-duty officer who was convicted of shooting a Marine veteran outside a bar.

"This is part of a decades-long, growing frustration over the extent to which police in Baltimore have adopted a highly militarized approach to policing residents of our city," said Sonia Kumar, a staff attorney with the American Civil Liberties Union of Maryland, which brought a 2006 lawsuit to change some police practices here.

On Friday, the police said Gray should have received medical treatment immediately at the scene of the arrest, and confirmed he was riding in a van unbuckled, a violation of department policy.

"Over the years, we have had a number of incidents that have tarnished this badge and the reputation of this department," said Police Commissioner Anthony Batts, adding, "I have been a reform commissioner."

The commissioner said he had fired 50 officers for misconduct, reduced excessive force and improved training since taking over the department in 2012. He dismissed calls for his resignation, saying, "That’s not going to happen."

Because there is no national database of police-involved killings, it is difficult to draw clear conclusions about the use of deadly force by the Baltimore police. However, a New York Times analysis of some Justice Department data suggests that Baltimore police officers have killed more people than the police have in a number of other cities similar in size.

Baltimore police officers killed 127 people over two decades ending in 2012, with a marked uptick in 2007 and 2008, according to the Justice Department’s voluntary survey of justifiable homicides by police. The police in Las Vegas, who cover that metropolitan area with a similar force, killed 100 people over the same period.

In other similar cities that reported to the survey each year, including Oklahoma City, Memphis, and Seattle — where the Justice Department found in 2011 a "pattern and practice of excessive force" — none reported more than half the number in Baltimore.

Last year, The Baltimore Sun reported that taxpayers had paid $5.7 million since 2011 in judgments or settlements in 102 lawsuits alleging police misconduct. A. Dwight Pettit, a lawyer who specializes in police misconduct and represents Tyrone West’s family in a wrongful-death suit against the city, said he had "20 open cases right now," and was flooded with requests for representation.

Gray was not the first black man in Baltimore to emerge from a police van with a spinal cord injury. Jeffrey Alston, who became paralyzed from the neck down after a van ride, settled for $6 million in 2004. The following year, Dondi Johnson Jr., also paralyzed after a van ride, won a jury award of $7.4 million, though it was reduced on appeal.

Kerry D. Staton, the lawyer who handled both cases, said Johnson, like Gray, had not been belted in. Staton said officers had intentionally given Johnson what is known as a "rough ride, where he was thrown across the vehicle into the opposite wall and broke his neck."

Civil rights advocates and some elected officials here trace the tensions to "zero-tolerance policing," a crime-fighting strategy championed by Martin O’Malley, the former governor and a potential Democratic candidate for president, when he was the mayor of Baltimore from 1999 to 2007. Aides of O’Malley note that on his watch, the number of annual homicides dropped below 300 per year for the first time in more than a decade, and that violent crime in Baltimore dropped by 41 percent. Steve Kearney, a top aide to O’Malley when he was the mayor, described the policies as "appropriate for the time."

But zero-tolerance policing led to mass arrests of people for small infractions, as well as intense "community frustration," Kumar of the ACLU said. "Countless innocent people," she added, "were getting caught up in this dragnet style of policing."

In 2006, the ACLU and the NAACP sued Baltimore, alleging a broad pattern of abuse. The city settled in 2010 for $870,000 and publicly abandoned zero-tolerance policing. But people here say tensions persist.

In Gray’s case, the police acknowledged that three bicycle officers pursued him after a lieutenant "made eye contact" with him and he ran away. The Gray family’s lawyer, William Murphy Jr., has said Gray was pursued for "running while black." That is one reason his case has provoked such an uproar here.

"I just want them to be able, when they come into our community, not to be afraid of us," said Darlene Cain, a nurse’s assistant who founded an advocacy group, Mothers on the Move, after her son Dale Graham was killed by a Baltimore police officer in 2008. "Be able to say, ‘Hello, good morning.’ Don’t just sit in your car and look at us like we’re the next person you want to lock up."

Rawlings-Blake said that on her watch, citizen complaints alleging police discourtesy or excessive force had declined. She also testified before the Maryland General Assembly in favor of a bill that would make it easier to investigate officer misconduct; the measure, strongly opposed by the police union, never made it out of committee.

But two measures — one doubling the maximum awards in civil lawsuits by those injured by the police, and another requiring reporting of police-involved killings in Maryland — did pass, and Gov. Larry Hogan said he intended to sign them into law.

Against this backdrop, Baltimore residents like Jones had become increasingly vocal in recent years, long before the fatal shooting of Michael Brown by the police in Ferguson, Missouri, last year put police treatment of black men on the national radar screen.

Jones’ brother, West, died after a struggle with Baltimore police officers during a July 2013 traffic stop. The state medical examiner ruled that he had died from a heart attack brought on by an underlying heart ailment and dehydration from the summer heat, and the city prosecutor declined to press charges.

But witnesses said West had been beaten; Jones said the funeral director "said my brother was beat so bad we’re going to need a closed casket." Rawlings-Blake commissioned an independent review, which concluded that while the police had not used excessive force, officers had made "tactical errors" that exacerbated the situation.

Jones and relatives have protested every Wednesday night, often in front of City Hall, for more than 80 weeks, vigils they call "West Wednesdays." Kumar, the ACLU lawyer, credits the family with raising awareness here and, now that national attention is on police-involved killings, setting the stage for the uproar over Gray’s death.

Pettit agrees.

"So much is happening," he said. "Lawyers like myself, community activists, for years we were screaming in the dark, and nobody was listening to us."

Sheryl Gay Stolberg, New York Times

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