NEW YORK (CNN/Money) - Shares of Citigroup tumbled Tuesday as congressional investigators charged that the No. 1 financial services company helped Enron manipulate its finances to deceive investors.

The losses were just the latest blow to Citigroup, which under the leadership of Sandy Weill became a $112 billion company with banking, insurance and brokerage services around the world.

Citigroup (C: Research, Estimates) shares tumbled $5.04 to $27 on the New York Stock Exchange, a drop of 15.7 percent. More than 121 million shares changed hands on the Big Board -- about eight times the stock's average daily volume -- a signal that big institutional investors may be losing faith in the company.

To be sure, it's been a tough year for all financial firms, not just Citigroup. But the New York-based financial services supermarket seems to be getting hit on a lot of fronts of late. The company's Salomon Smith Barney unit has been sued over how it doled out IPOs, for example, and the brokerage's best-known analyst, Jack Grubman, has come to symbolize conflicts of interest on Wall Street.

Citigroup's latest losses came as members of the Senate's Permanent Subcommittee on Investigations said that some banks, including Citigroup, helped Enron hide debt during the months before the energy trader went bankrupt.

Citigroup, along with J.P. Morgan Chase, has called the arrangements appropriate.

But wary investors have punished financial services companies for perceived connections to the widening corporate scandals that have helped pushed stocks to five-year lows.

"I think it's very serious," Kim Arthur, director of equity marketing at Banc of America Securities, told CNNfn's Halftime Report.

Investigators contend that the banks helped Enron report cash flow that was really debt. Arthur said that charge, if true, could hamstring the financial services companies that moved beyond pure lending and into more exotic transactions to drive profits.

"They [Citigroup] are under intense scrutiny, but Sandy Weill has proven himself to be a worthy competitor in the past," Arthur said.

Anton Schutz, portfolio manager at the $41 million Burnham Financial Services Fund, which owns Citigroup shares, said the selloff creates an opportunity for investors to buy an undervalued company.

"Is any of this stuff going to destroy the underlying value of Citigroup?" asked Schutz. "The answer is: No."

With $90 billion in equity capital, Citigroup, Schutz said, will have no problem paying any fines.

"People just have a lack of faith in corporate governance and business ethics," he said.

Doubts about the credibility of analyst research may also have weighed on Citigroup.

Grubman, the telecom analyst at Salomon Smith Barney, faced a grilling by Congress earlier this month over his close ties to WorldCom, the company that filled the biggest bankruptcy in history Sunday.

The regulatory arm of the National Association of Securities Dealers said Monday it told Grubman the agency is looking into why he kept a "buy" rating on shares of the now-bankrupt competitive local exchange carrier Winstar Communications Inc. as its financial difficulties mounted.

Merrill Lynch earlier this year paid $100 million to settle charges that its securities analysts touted dubious stocks to win business for their banking clients.

The SEC will consider a measure requiring analysts to certify that their picks aren't influenced by investment banking business.

As for Citigroup, more than 84 million Citigroup shares changed hands Tuesday afternoon, making it the most actively traded company on the New York Stock Exchange followed by J.P. Morgan Chase (JPM: Research, Estimates), whose shares tumbled 18 percent.

On Tuesday, Robert Roach, chief investigator for the Senate panel, said the banks may have helped Enron in return for big fees and favors in other deals.

"Enron's practice of using prepaid transactions to understate debt and overstate cash flow from operations made its financial statement look much stronger; that in turn helped Enron maintain its investment rate credit rating and support -- even boost -- its share price," Roach said

Investigators believe the deals with both banks helped Enron understate its debt in 2000 by $4 billion, or 40 percent, and overstate its cash flow by up about $1.7 billion, or about 50 percent.

"Hopefully we'll cut through the darkness and place appropriate level of responsibility on the banks who participated in these schemes," said Sen. Carl Levin, D-Mich., the chairman of the Senate subcommittee.

Citigroup, J.P. Morgan Chase and GE Capital were big lenders to WorldCom, which improperly accounted for more than $3.8 billion in expenses before its bankruptcy.

But as the biggest financial services company, Citigroup's tumble mirrors that of other tarnished stars. General Electric (GE: Research, Estimates), the biggest company by market value, and IBM (IBM: Research, Estimates), the No 1 technology company by sales, both have suffered amid investor distaste for large, complicated companies that came to typify the 1990s bull market.

Steve Yeary, market strategist at New Millennium Advisors, says it's too soon to buy Citigroup.

"Nothing good can come out of this," Yeary told CNNfn's The Money Gang. "I really think you should wait until the dust settles on this."