Researchers have long known that African-Americans are more likely to be arrested for marijuana possession than whites, even though studies have repeatedly shown that the two groups use the drug at similar rates.

New federal data, included in a study by the American Civil Liberties Union, now shows that the problem of racially biased arrests is far more extensive that was previously known — and is getting worse. The costly, ill-advised “war on marijuana” might fairly be described as a tool of racial oppression.

The study, based on law enforcement data from 50 states and the District of Columbia, is the most detailed of its kind so far. Marijuana arrests have risen sharply over the last two decades and now make up about half of all drug arrests in the United States. Of the more than eight million marijuana arrests made between 2001 and 2010, nearly 90 percent were for possession. There were nearly 900,000 marijuana arrests in 2010 — 300,000 more than for all violent crimes combined.

Nationally, African-Americans are nearly four times as likely to be arrested for marijuana possession as whites. The disparity is even more pronounced in some states, including Illinois, Iowa and Minnesota, where African-Americans are about eight times as likely to be arrested. And in some counties around the country, blacks are 10, 15 or even 30 times as likely to be arrested.