Regulators across the country are confronting a wave of investor fraud that is saddling retirement savers with steep losses on complex products that until a few years ago were pitched only to the most sophisticated investors.

The victims are among the millions of Americans whose mutual funds and stock portfolios plummeted in the wake of the financial crisis, and who started searching for ways to make better returns than those being offered by bank deposits and government bonds with minuscule interest rates.

Tens of thousands of them put money into speculative bets promoted by aggressive financial advisers. The investments include private loans to young companies like television production firms and shares in bundles of commercial real estate properties.

Those alternative investments have now had time to go sour in big numbers, state and federal securities regulators say, and are making up a majority of complaints and prosecutions.

“Since the crisis, we’ve seen more and more people reaching out into different types of exotic investments that are a big concern to us,” said William F. Galvin, the Massachusetts secretary of the commonwealth.

Last Wednesday, Mr. Galvin’s office ordered one of the nation’s largest brokerage firms, LPL Financial, to pay $2.5 million for improperly selling the real estate bundles, known as nontraded REITs, or real estate investment trusts, to hundreds of state residents from 2006 to 2009, in some cases overloading clients’ accounts with them.

LPL said it agreed, as part of the settlement, to reform its process for selling such alternative investments.

There are few good statistics on the extent of the problem nationally. But cases are mounting in the offices of regulators like A. Heath Abshure, the securities commissioner in Arkansas, where a majority of the 66 open securities cases involve complex investments sold to less sophisticated investors looking for a steady return.

J. Bradley Bennett, chief of enforcement at the Financial Industry Regulatory Authority, or Finra, Wall Street’s self-regulatory group, said that for the last two years, 10 staff members have looked at the “proliferation of these products, to understand how they are being sold.”

“It’s got our attention,” he said. “We recognize the trends.”

Brokers promoting bad investments to unsophisticated investors is nothing new. But while the easy prey used to be people looking to get rich quick, the pool has widened to include savers looking for ways to earn the kind of income once reliably available from traditional investments.

Regulators are warning investors that the dangers are unlikely to recede, given the Federal Reserve’s pledge to keep interest rates near zero and the push among financial firms to earn more revenue from so-called alternative investments marketed to retail investors. Brokers are eager to sell these investments because they often bring in higher commissions than standard mutual funds and stocks.

The money that retail investors have in alternative investments in the United States, ranging from baskets of commodities to mutual funds that employ sophisticated trading, more than doubled from 2008 to 2012, to $712 billion from $312 billion, according to McKinsey & Company. Many of the products hold out the promise of higher returns while ostensibly being immune to the volatility of stock markets.

The phenomenon of investors’ actively moving money in pursuit of higher interest rates, known as chasing yield, is reverberating through the economy. Jeremy C. Stein, a Federal Reserve governor, said in a speech on Thursday that he worried that investors desperate for yield could be creating a bubble in widely available investments like junk bonds.

Mary Beck, a furniture business consultant in Pasadena, Calif., said that in 2008, as the stock investments in her husband’s I.R.A. began to fall quickly, the couple moved $470,000 to a new product recommended by their broker.

While the offering was unfamiliar — part ownership in a fleet of luxury cars — Ms. Beck bought the pitch because her broker had been around for years, and the product offered what seemed to be a modest annual interest rate of 7 percent.

“We knew that 12 percent wasn’t realistic, but 7 percent seemed realistic,” Ms. Beck said. “To us, it was a very conservative way to ensure that we’d increase our savings.”

Soon after they stopped receiving interest payments, the Becks lost their money when the venture went bankrupt in 2012. Ms. Beck and her husband have been reconfiguring their retirement and are planning to work longer.