CHARLOTTE, N.C. — Duke Energy's shares plunged to a new 52-week low Monday as three brokerage firms downgraded its stock, saying they were concerned about federal investigations into the utility's energy trading practices.

After falling below $20 earlier Monday, or by more than 20 percent, Duke's shares regained ground at the end of the day to close at $23.17, down $1.05, or 4.2 percent. Volume was very heavy with nearly 30 million shares trading hands.

Duke Energy said Friday that it had received subpoenas from federal authorities and was responding to the requests for information about its energy trading practices.

Meanwhile, Duke Energy spokesman Terry Francisco confirmed Monday that a senior manager in Duke's Houston-based energy trading operations has been put on paid administrative leave while the investigators look into the company's trading practices.