KABUL—More than $3 billion in cash has been openly flown out of Kabul International Airport in the past three years, a sum so large that U.S. investigators believe top Afghan officials and their associates are sending billions of diverted U.S. aid and logistics dollars and drug money to financial safe havens abroad.

The cash—packed into suitcases, piled onto pallets and loaded into airplanes—is declared and legal to move. But U.S. and Afghan officials say they are targeting the flows in major anticorruption and drug trafficking investigations because of their size relative to Afghanistan's small economy and the murkiness of their origins.

Officials believe some of the cash, if not most, is siphoned from Western aid projects and U.S., European and NATO contracts to provide security, supplies and reconstruction work for coalition forces in Afghanistan. The North Atlantic Treaty Organization spent about $14 billion here last year alone. Profits reaped from the opium trade are also a part of the money flow, as is cash earned by the Taliban from drugs and extortion, officials say.