New Delhi, July 24 -- UnitedHealth stock plunged on Thursday after the company disclosed it was under investigation by the US Department of Justice.

The healthcare conglomerate said it was complying with both criminal and civil requests from the federal agency.

At 12:19 PM EDT, UnitedHealth stock was down 4.15% at $280.38.

So far this year, the stock shed over 40% of its value.

"(UnitedHealth) has a long record of responsible conduct and effective compliance," the company said in a Securities and Exchange Commission filing.

The DOJ was investigating UnitedHealth's Medicare business, a US government program that covers medical costs for individuals aged 65 and older and those with disabilities, reported Wall Street Journal earlier thi...