By Jessica Silver-Greenberg August 6, 2013 4:02 pm August 6, 2013 4:02 pm

The Justice Department sued Bank of America on Tuesday, accusing the bank of defrauding investors by vastly understating the risks of the mortgages backing about $850 million in securities.

The lawsuit adds to the hefty legal burden of the bank, which has been badly battered by mortgage-related losses and litigation since the financial crisis. The great bulk of those troubles stem from the bank’s 2008 acquisition of the subprime lender Countrywide Financial.

Yet the latest litigation centers on Bank of America’s own homegrown mortgage operations. And the loans at issue were represented as prime jumbo mortgages — at the time, 2007, loans of more than $417,000 for a single-unit dwelling — rather than subprime mortgages, which were at the heart of the mortgage crisis.

Those new elements were woven into a familiar narrative as the Justice Department’s lawsuit portrayed the bank’s mortgage operations as emblematic of Wall Street’s reckless practices in the heady days before the financial crisis.

Under pressure to generate profits, the lawsuit said, Bank of America pushed employees to churn through mortgage evaluations. The instructions for slipshod standards emanated from the upper echelons of the bank, the lawsuit said.

One employee, according to the lawsuit, said that her job was to “basically validate the loans,” rather than to comb through them to spot flaws. The goal, the employee said, was to get through mortgage applications swiftly. She was told by her superiors, prosecutors said, to “keep her opinions to herself.”

In the end, the prime mortgages turned out to be far more dangerous than investors were led to believe, the lawsuit contends. Bank of America misrepresented the quality of the loans, and five investors lost about $100 million, the government said.

A spokesman for Bank of America contested the government’s characterization of its conduct.

“These were prime mortgages sold to sophisticated investors who had ample access to the underlying data and we will demonstrate that,” Lawrence Grayson, a spokesman, said in a statement.

Bank of America disclosed last week in a corporate filing that it was bracing for an action from the Justice Department. The Securities and Exchange Commission filed a parallel action on Tuesday.

The lawsuit is the latest salvo from President Obama’s federal mortgage task force. Eric H. Holder Jr. the United States attorney general, said it was “the latest step forward in the Justice Department’s ongoing efforts to hold accountable those who engage in fraudulent or irresponsible conduct.”

Still more than five years after the housing market imploded, federal prosecutors are still working to pin blame for problems that helped drive the mortgage boom and then its demise. It is an effort that has generated some frustration among the American public, fed up with the lack of criminal prosecutions since the financial crisis, and a slew of civil lawsuits, some of them seemingly repetitive, aimed at banks.

For Bank of America, the lawsuit on Tuesday follows a case from federal prosecutors in New York. In that civil suit last year, federal prosecutors claimed that Bank of America, through its Countrywide Financial unit, defrauded the government by producing loans at such a fast clip that controls were ignored.

The Justice Department’s lawsuit on Tuesday also highlights a lack of controls at the bank. An “unprecedented” number of the loans packaged and sold to investors came from mortgage brokers, it said. As a result, prosecutors said, the loans were shaky, at best.

At the time, Bank of America’s chief executive, Kenneth D. Lewis, described loans that came through such wholesale channels as “toxic waste,” according to the lawsuit.

Once the mortgages were originated, the government said, Bank of America deliberately kept investors blind to the risks associated with the ensuing securities. Bank of America, the government said, never told investors that some of the mortgages included were so-called stated income and stated asset loans, meaning that the bank never verified the income the borrowers claimed to have.

Prosecutors said that investors were also deprived of critical knowledge about the underlying mortgages, like the fact that 22 percent of the borrowers were self-employed. The investments at issue were sold in 2008 to five investors, including the Federal Home Loan Bank of San Francisco and Wachovia Bank, which has since been acquired by Wells Fargo.

Without quality controls, the government said, trouble ensued. The number of borrowers that fell behind on their payments was “abnormally high,” according to the lawsuit. The turbulence “cannot be explained solely by the downturn in the real estate market” alone. According to the government’s assessment, at least 23 percent of the mortgages had defaulted or were delinquent as of June 2013.

The lawsuit cites 2007 e-mails from the bank’s own securities traders expressing frustration about the quality of mortgages. One trader wrote, some were “like a fat kid in dodge ball, these need to stay on the sidelines.”

While the accusations add to the pressure on Bank of America, which has been working to move past the crisis, previous government lawsuits against it that began with much fanfare have lost some of their momentum.

In the United States attorney’s lawsuit against Bank of America that was filed in October, for example, prosecutors originally said that the problematic home loan program known as the “hustle” was kept alive by Bank of America through 2009.

But, last month, the government scaled back those accusations. The program lasted through May 2008, not through December 2009, the government said in a court filing.