The Securities and Exchange Commission has launched a formal investigation into an apparent miscalculation of net rental income last summer by Apartment Investment & Management Co. Earlier this month, AIMCO received a letter from the SEC stating that the “informal” investigation had been upgraded to “formal” status. The giant apartment REIT owns or manages roughly 300,000 apartment units throughout the nation.

Last November, AIMCO disclosed that its net rental figures for last July, August and September were wrong. Instead of the total net income of $267.2 million for that was originally reported for that period, AIMCO revised that figure down to $265.2 million. Average monthly rent per unit was also lowered to $702 from $707.

Neither the SEC nor AIMCO has commented extensively about the investigation. However, an AIMCO spokeswoman was quoted by The Wall Street Journal as saying she doesn’t believe the inquiry will have a material effect on earnings.

The general view is that there will have to be some significant restructuring at AIMCO, says Barry Vinocur, editor of Realty Stock Review. “Investors are getting very impatient with AIMCO.”

AIMCO shares have fallen nearly 8% over the past month. The stock traded at a 52-work low on Tuesday. Shares were trading at $30.93 at noon today, down 0.61% for the week.