Defense Department documents show the United States has sold ammunition, combat vehicle parts, communications equipment, helicopters and an unidentified missile system to Bahrain.
The documents don't clarify whether the arms have been delivered or how much the materials are worth.
ProPublica said U.S. officials have said they are selling the arms to the country only for external defense, but many are worried the weapons could be used against civilian protesters.
ProPublica cited reports Bahrain used U.S.-made helicopters to fire on protesters.
A U.S. team specialized in training foreign militaries was also in Bahrain to help with the use of Blackhawk helicopters, ProPublica reported.
ProPublica said the Obama administration has stood by Bahrain's Sunni ruling family during the Shiite protests.