By Frances Harrison
BBC News, Tehran
US treasury officials are investigating whether American basketball players who were paid to play for Iranian teams have violated US sanctions.
American basketball players are popular with Iranian fans
Under the sanctions regime, a special licence is required by any American citizen providing services to Iran.
An Iranian adviser to the basketball teams said the US players did not know they were violating any restrictions.
Last year, 20 Americans were contracted to play for Iranian teams. They could now face fines of $50,000 (£26,700).
Basketball is a popular sport in Iran, and many saw the Americans' involvement as a rare example of sport overcoming political tensions.
However, this co-operation looks set to end, and there are now only two American players left in Iran.
Letter of the law
Existing US sanctions against Iran prevent any American citizen from engaging in any transaction or providing any service to Iran or its government without a special licence from the Office of Foreign Assets Control.
But diplomats in Iran say these regulations have never been implemented before against individual sportsmen.
They point out that if the letter of the law was followed, then no American student or dual national in Iran could even give English classes for money.
The US treasury department has started contacting American basketball players who have played for Iranian teams, asking for details of their contracts, which Americans helped them go to Iran, and information about any other American players in Iran.
US treasury spokeswoman Molly Millerwise said it was standard procedure when a violation was suspected to enter into a discussion with the parties, but said she could neither confirm nor deny investigations into individual cases.
She said the penalty could be up to $50,000 (£26,700).
But an Iranian adviser to the basketball teams said: "Although everything was developing on a sound footing and cultural environment, the US players were placed under investigation without their knowledge that they were violating the sanctions regime."