The European Commission Tuesday said it is extending its antitrust investigation into Dutch government support for troubled bank ING Group NV (ING) on concerns over valuation.

The commission also said the aid measure could stay in place for the duration of the probe.

The commission in mid-March cleared the Dutch government to help ING handle illiquid assets for six months.

Under the plan, the government bought the right to cash flows on 80% of ING's $39 billion portfolio of U.S. mortgage loans, mostly consisting of Alt-A mortgages, for about $28 billion. Alt-A mortgages are a category of loans rated between prime and sub-prime.

The commission has said the price paid for the cash flows wasn't "conservative enough," and found that a significant proportion of securities were valued above the purchase price.

If the commission finds that its concerns are justified, ING will have to increase payment to the Dutch government.

-By Peppi Kiviniemi, Dow Jones Newswires; +3227411483; peppi.kiviniemi@dowjones.com