Pimco, an investment firm, considered acquiring the 850-property portfolio before its eventual sale in April 2014.
In a statement Pimco said it was contacted on "an unsolicited basis by third parties with a proposal relating to the potential purchase".
It did not identify the third parties.
The statement went on: "Pimco assessed the opportunity and followed its usual due diligence process, and as a result decided not to proceed with, or agree to any arrangement with those third parties."
Weeks later, Nama began a formal sale process that led to another US firm, Cerberus Capital Management, buying the portfolio for more than £1bn.
Nama has said it was "fully satisfied" that the process had delivered the "best possible return", with Cerberus being the highest bidder.
In its statement, Pimco said it did not take part in the tender process.
A potential US buyer of the Northern Ireland Nama property portfolio has said it was pitched a sale proposal by way of an "unsolicited approach from third parties".