On a 12-month basis, the CPI for all items rose 1.7 percent before seasonal adjustment. The index for all items minus food and energy, called core prices, rose 0.2 percent.
Over a 12-month period core prices, which are monitored intently by the U.S. Federal Reserve, rose 2.3 percent, slightly above the central bank's target of 2 percent or lower.
The energy index pushed the decline with prices dropping 4.3 percent in May. Gasoline prices, separately, fell 6.8 percent.
Energy prices fell, dropping 3.9 percent on an annual basis, the first 12-month decline for the category in 32 months.
The food index was flat in May, but posted a 2.8 percent increase over 12 months.
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