The last century's racial disturbances have a common cause: police brutality.

The killing of an unarmed black man by a white policeman, everyone agrees, triggered last week's riots in Cincinnati. The fourth police slaying of a black Cincinnatian since November, and the 15^th since 1995, Timothy Thomas' death uncorked a gusher of pent-up anger toward a police force that black citizens view as racist, tyrannical, and brutal. So striking is the pattern of discriminatory behavior among Cincinnati's men in blue that even Attorney General John Ashcroft, not previously known as a friend to African-Americans, announced that the Justice Department will investigate the city's policing practices.

Some critics cite "racial profiling" as the cause of the Cincinnati riots, and while that euphemism may be new, police discrimination against blacks long predates it. Indeed, for all the anguished searching for the roots of the Cincinnati riots and those of the numerous disturbances that preceded it last century, there's no great mystery about their cause. Almost invariably, an instance (real or rumored) of police brutality or abuse of power, capping a history of tension with a city's black community, has provoked this country's major riots. Consider:

• Chicago, 1919. On a hot July afternoon, a black teen-ager named Eugene Williams went swimming in a customarily "white" area of Lake Michigan. Angry white beach-goers pelted him with stones until he drowned. When police refused to apprehend the culprit and instead arrested a black onlooker, blacks, who had long considered the police to be insensitive to their concerns, attacked the officers. Whites retaliated, ushering in four days of brutal violence in which policemen at times abetted the white-on-black assaults.

• Harlem, 1935. On March 19, department-store employees apprehended a 16-year-old shoplifter and whisked him off to be beaten in the store's basement. An outraged crowd massed on 125^th Street, as rumors circulated that that police had broken the arms of a black woman who tried to help the boy—made plausible, again, because of the police's record of hostility to Harlem's blacks. Rioting ensued for the next day, resulting in two deaths.

• Watts, 1965. On Aug. 11, police stopped motorist Marquette Frye on Avalon Street in the Los Angeles ghetto. Frye resisted arrest, a crowd gathered, and more policemen came. When some of them roughly manhandled Frye's mother, the story took hold that cops had kicked a pregnant woman in the belly, further angering the mob. Violence reigned for four days, with 34 people killed.

• Detroit, 1967. On July 23, police raided a black drinking and gambling club, arresting some 80 habitués. Hordes of neighborhood blacks massed outside the establishment, first taunting the policemen, then throwing stones. A swarm of police cruisers swooped in but made no serious effort to disperse the crowd. Spontaneous looting and arson followed, and five days later, 43 people lay dead, many killed by police or National Guardsmen.

• Miami, 1980. On May 17, a Tampa jury acquitted four Miami policemen in the killing of Arthur McDuffie, a black businessman, whom they had beaten to death after chasing his motorcycle—the latest in a series of cases of police brutality. Mayhem followed, with black mobs attacking whites and white policemen retaliating with deadly gunfire. Eighteen people lost their lives.

This list represents just a smattering of the numerous civil disturbances in 20^th-century America sparked by friction between policemen and black citizens. It doesn't include the riots in East St. Louis in 1917, Detroit in 1943, Harlem in 1964, Los Angeles in 1992, St. Petersburg in 1996, and the dozens of mid-1960s outbursts around the country. But there, too, cops were a critical catalyst in the riots.

History repeated itself in another way as well: Almost all these disturbances generated an official commission that sought to determine the cause of the violence. These commissions invariably discussed the obvious systemic social problems—the woeful state of urban housing, education, and job opportunities for blacks. But they also routinely cited police actions against African-Americans as a key factor in the uprisings.

After the 1935 Harlem riot, a mayoral commission discovered long-standing discrimination by police against blacks. After Detroit's riots in 1943, Thurgood Marshall of the NAACP wrote a report titled "The Gestapo in Detroit" that rebuked the city for failing to face up to its police department's consistently unequal treatment of the races. The so-called Kerner Commission, empanelled by Lyndon Johnson in 1967, is usually remembered for noting the social chasm between black and white Americans; less often recalled is the report's chapter describing "deep hostility between police and ghetto communities as a primary cause of the disorders surveyed by the commission."