The Justice Department sued Bank of America on Tuesday, accusing the bank of defrauding investors by vastly underestimating the quality of mortgage-backed securities.

The lawsuit is the latest action by President Barack Obama’s federal mortgage task force that has vowed to hold Wall Street accountable for misconduct in the packaging and sale of mortgage securities during the housing boom.

Bank of America, the Justice Department said, cloaked the risk associated with $850 million worth of securities backed by residential mortgages. In a corporate filing last week, Bank of America said it was bracing for the action.

Eric Holder, the U.S. attorney general, said the lawsuit was “the latest step forward in the Justice Department’s ongoing efforts to hold accountable those who engage in fraudulent or irresponsible conduct.”

As Bank of America assembled securities in 2008, the government claimed, the bank ignored that more than 40 percent of the mortgages included did not meet underwriting guidelines. Even though Bank of America knew about the troubled mortgages, the government said, the bank sold the securities anyway.

Unlike other lawsuits, this case zeros in on prime mortgages, rather than subprime loans.

Prosecutors portrayed Bank of America’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 pressured employees to churn through mortgage evaluations. The instructions for slipshod standards stemmed from the top 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.