Police are consulting with the Crown Prosecution Service over the land sale that paved the way for a Tesco store in Corby.
Two plots at St James industrial estate were sold by Corby Council to developers in 2010 for a total of £3.8m.
But an audit three years later found the land could have been worth up to £13.3m.
The report recommended that the council investigate the legal implications of the sale and its potential breach of Section 123 of the Local Government Act, which prohibits the disposal of land by local authorities for a price less than the best that can be reasonably obtained.
Questions were raised by several councillors, including leader of the opposition Conservatives on Corby Council Cllr Rob McKellar, over whether the sale was illegal.
A Northamptonshire Police spokesman said: “An inquiry has taken place to establish if any criminal offences have been committed.
“As part of that inquiry we are currently consulting with the CPS, which is a formality.”
The CPS will now decide if any charges are to be brought as a result of the investigation.