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.

The amount declared as it leaves the airport is vast in a nation where the gross domestic product last year totaled $13.5 billion. More declared cash flies out of Kabul each year than the Afghan government collects in tax and customs revenue nationwide. "It's not like they grow money on trees here," said a U.S. official investigating corruption and Taliban financing. "A lot of this looks like our tax dollars being stolen. And opium, of course."

Most of the funds passing through the airport are being moved by often-secretive outfits called "hawalas," private money transfer businesses with roots in the Muslim world stretching back centuries, officials say.