The investigation has already led to a $80m (£45m) penalty being paid by the Dutch bank ABN Amro which, along with Swiss giant UBS, has cut back its ties with Tehran under pressure from Washington.
The US government has been attempting to put pressure on Iran over its nuclear programme, which has been upgraded since the election in August last year of President Mahmoud Ahmadinejad.
The International Atomic Energy Agency is considering a call to report Iran to the United Nations Security Council over its nuclear programme. This could lead to wide-ranging sanctions being applied against the country. However, the US already has sanctions in place against Iran, as well as prohibitions against dealing with organisations it believes are involved in terrorism.
The Justice Department is investigating whether companies doing business with Iran might have broken those rules.
In 2004, the chairman of the US Senate Finance Committee, Charles Grassley, wrote to the Treasury Department asking whether three US corporations - GE, Halliburton and Conoco-Phillips - might be breaking sanctions. All three have reduced their operations in Iran.
A new line of investigation has focused on banking relationships. In December, ABN Amro paid $80m in penalties after it was found to have violated rules on dealing with Iran and Libya, while UBS paid $100m in 2004 after admitting breaches of rules on transactions with Iran, Libya, Cuba and Serbia.
Both ABN Amro and UBS have reduced their Iran operations, as has Credit Suisse. But it has emerged that the Justice Department is probing three more banks - HSBC, Standard Chartered and France's BNP Paribas.
HSBC has a representative office in Tehran with 30 staff. It said that it was aware of the Justice Department's actions against other banks but had no plans to reduce its Iranian operations.
Standard Chartered also has a representative office in Tehran, and recently opened a branch at Kish Island, a free trade area and resort being developed by the Iranians. It too said it had no plans to withdraw from the country.
The Royal Bank of Scotland also operates in Iran, but it is not believed to be the subject of any US regulatory interest.
A senior banking source expressed unhappiness about the Justice Department probe. He said that as far as he was aware, all the UK banks were operating fully within British laws and banking regulations. They also had to comply with regulatory rules in Dubai, which is the regional hub for financial services.
"If the US wants to apply its laws to banks not based in America, then that will only cause regulatory confusion," he said.
RBS was recently at the centre of controversy in the US because it maintains an account for a UK-based Palestinian charity that the US government claims is a terrorist front.Reuse content