The US government has spent about $55bn on rebuilding in Afghanistan since 2001 but cannot easily show how the money was spent, a government watchdog says.
The special inspector general's office for Afghanistan reconstruction talked of a "confusing labyrinth" of spending.
It said some 7,000 contractors received $17.7bn from 2007-09 but data prior to 2007 was too poor to be analysed.
It is the first comprehensive audit of US spending in Afghanistan since US-led troops ousted the Taliban in 2001.
According to the report, US government agencies are not tracking Afghan contracts in a shared database and cannot easily show where the money went.
The BBC's Quentin Sommerville in Kabul says record-keeping has been so poor that most of the money has not been properly recorded.
The Pentagon, state department and USAID "are unable to readily report on how much money they spend on contracting for reconstruction activities in Afghanistan", said the report from the special inspector general's office, which was set up by Congress.
It was also not clear who had received money disbursed by the three agencies, which are the biggest US spenders on Afghan reconstruction.
Pentagon contracts worth $11.5bn for construction, supplies and logistics in Afghanistan went to more than 6,615 contractors between 2007 and 2009, the audit found. Half of that money went to just 41 contractors.
USAID spent $3.8bn during that time and the state department $2.4bn.
"The audit shows that navigating the confusing labyrinth of government contracting is difficult, at best," according to the watchdog.
It said there had been little co-ordination within and between US government agencies. The three agencies mentioned above, for example, do not separate their spending in Afghanistan from other US-funded projects around the world.
"If we don't even know who we're giving money to, it is nearly impossible to conduct systemwide oversight," the inspector general, Arnold Fields, said.
US special envoy Richard Holbrooke has voiced similar concerns in the past, talking of an "ununified" effort by the US, the UN and hundreds of other countries and aid agencies in Afghanistan.
According to the inspector general's audit, the largest contract between 2007 and 2009 was with US company DynCorp. It received about $1.8bn for police training and counter-narcotics work in Afghanistan.
A Kabul construction company received nearly $700m to build offices and barracks.
In a separate report, the inspector general found that six buildings constructed for the Afghan national police - which cost the US taxpayer $5.5m - were unusable.
The quality of construction was so bad that the sites in Helmand and Kandahar could collapse in an earthquake, it reported.