To conclude that India has "significantly" reduced its oil imports from Iran, the US collected figures from multiple sources, including those from the Indian government and available in public domain.
The findings paved the way for Washington to exempt New Delhi from sanctions.
The US on Monday said it would exempt India and six other nations from financial sanctions because they have significantly cut purchases of Iranian oil.
"I have made the determination that seven economies—India, Malaysia, Republic of Korea, South Africa, Sri Lanka, Turkey and Taiwan--have all significantly reduced their volume of crude oil purchases from Iran," Secretary of State Hillary Clinton said in a statement.
Such a statement from Clinton came as the US President, Barack Obama, was of the view that there was enough oil in the global market for the countries to reduce their dependence on Iranian oil.
"The process of reaching any decision on a determination on exception takes into account a range of data sources that's pulled together by agencies across the United States government, including the Department of Energy, the Energy Information Administration, the Department of Treasury, the Department of State, the intelligence agencies," a senior administration official told reporters.
"We review commercially available data. We review data that is made available at different points in time by importers themselves," the official said speaking on condition of anonymity.
The official was responding to the question on what is the percentage of oil reduction of India from Iran, which he did not respond to.
Informed sources said India's reduction has been in the range of 15 to 20%.
"The Indian government also took steps to publish data on its previous imports through process of parliamentary questions. These were published on the website of the lower House of Parliament. It was based on taking into account all of these factors that we reached our decision," the official said.
Notably India had not sought any exemption from the US sanctions nor had it applied for it, with the official argument that India does not believe in unilateral sanctions and it strictly abide and complies by multilateral sanctions like those by the UN Security Council.
Experts believe a decision otherwise by the Obama Administration would have had significant adverse impact on India-US relationship and would have definitely cast a shadow on the strategic dialogue.
Clinton issued the statement in this regard hours before the External Affairs Minister, SM Krishna, was to arrive in Washington for third India-US Strategic Dialogue.
In her statement, Clinton said as a result of her determination, she will report to the Congress that relevant sanctions will not apply to financial institutions of these countries.
"We have implemented these sanctions to support our efforts to prevent Iran from acquiring a nuclear weapon and to encourage Iran to comply with its international obligations," she said.
"Today's announcement underscores the success of our sanctions implementation. By reducing Iran's oil sales, we are sending a decisive message to Iran's leaders: until they take concrete actions to satisfy the concerns of the international community, they will continue to face increasing isolation and pressure," Clinton had said on Monday.