WASHINGTON — Black Americans were nearly four times as likely as whites to be arrested on charges of marijuana possession in 2010, even though the two groups used the drug at similar rates, according to new federal data.

This disparity had grown steadily from a decade before, and in some states, including Iowa, Minnesota and Illinois, blacks were around eight times as likely to be arrested.

During the same period, public attitudes toward marijuana softened and a number of states decriminalized its use. But about half of all drug arrests in 2011 were on marijuana-related charges, roughly the same portion as in 2010.

Advocates for the legalization of marijuana have criticized the Obama administration for having vocally opposed state legalization efforts and for taking a more aggressive approach than the Bush administration in closing medical marijuana dispensaries and prosecuting their owners in some states, especially Montana and California.