WASHINGTON/NEW YORK (Reuters) - The U.S. government on Tuesday filed two civil lawsuits against Bank of America that accuse the bank of investor fraud in its sale of $850 million of residential mortgage-backed securities.

The lawsuits are the latest legal headache for the second-largest U.S. bank, which has already agreed to pay in excess of $45 billion to settle disputes stemming from the 2008 financial crisis.

While most of the cases Bank of America has already confronted pertain to its acquisitions of brokerage Merrill Lynch and home lender Countrywide, the lawsuits filed on Tuesday pertain to mortgages the government said were originated, securitized and sold by Bank of America's legacy businesses.

The residential mortgage-backed securities at issue, known as RMBS, were of a higher credit quality than subprime mortgage bonds and date to about January 2008, the government said, months after many Wall Street banks first reported billions of dollars in write-downs on their holdings of subprime mortgage securities.

The Justice Department and the U.S. Securities and Exchange Commission filed parallel lawsuits in U.S. District Court in Charlotte, North Carolina, accusing Bank of America of making misleading statements and failing to disclose important facts about the pool of mortgages underlying a sale of securities to investors in early 2008.

The investors included the Federal Home Loan Bank of San Francisco and Wachovia Bank National Association, the Justice Department lawsuit said.

Bank of America, which is based in Charlotte, responded to the lawsuits with a statement: "These were prime mortgages sold to sophisticated investors who had ample access to the underlying data, and we will demonstrate that.

"The loans in this pool performed better than loans with similar characteristics originated and securitized at the same time by other financial institutions. We are not responsible for the housing market collapse that caused mortgage loans to default at unprecedented rates and these securities to lose value as a result."

Bank of America shares fell 1.1 percent to close at $14.64 on the New York Stock Exchange following news of the lawsuits, which were filed late in the afternoon.

Bank of America had warned in a securities filing on Thursday about possible new civil charges linked to a sale of one or two mortgage bonds.

According to the lawsuits, Bank of America made misleading statements and failed to disclose important facts about the mortgages underlying a securitization named BOAMS 2008-A. More than 40 percent of the 1,191 mortgages in the securitization did not comply with the bank's underwriting standards, according to the complaint.

"These misstatements and omissions concerned the quality and safety of the mortgages collateralizing the BOAMS 2008-A securitization, how it originated those mortgages and the likelihood that the 'prime' loans would perform as expected," the Justice Department said in its statement.

Threats of costly mortgage litigation have been dogging Bank of America for years.

"It has been shown repeatedly that the origination process at Bank of America and its subsidiaries failed to live up to their own internal guidelines and the resulting loans did not reflect the way they were characterized to investors," said Donald Hawthorne, a partner at Axinn Veltrop & Harkrider LLP, who has represented monoline insurers and RMBS investors in suits against mortgage originators, including Countrywide, relating to mortgage securities.

In 2011, the bank's shares fell more than 20 percent in a single day after American International Group filed a $10 billion lawsuit accusing the bank of mortgage fraud.

Weeks later, Warren Buffett's Berkshire Hathaway Inc swooped in with a $5 billion investment to shore up confidence in the bank. Since then, Bank of America's stock has more than doubled as the bank has announced agreements to settle major disputes, and investors have regained confidence in its outlook.

Among major deals, the company agreed to an $8.5 billion settlement with mortgage-backed securities investors, a $1.6 billion settlement with bond insurer MBIA Inc, and a settlement worth more than $10 billion with Fannie Mae, the government-controlled mortgage finance provider.

The lawsuit signals the federal government's willingness to pursue litigation challenging banks securitizations and marketing practices even as the financial crisis recedes further into the past.

The Justice Department's lawsuit was brought under the Financial Institutions Reform, Recovery and Enforcement Act, a savings-and-loan-era law that federal prosecutors have revived in recent years to continue pursuing civil fraud charges against financial institutions. It has a 10-year statute of limitations, double the deadline under other securities fraud laws.

The U.S. attorney's office in Manhattan brought a separate suit against Bank of America under that act last October over losses that Fannie Mae and Freddie Mac suffered on loans the government said were deficient.

Attorney General Eric Holder said in a statement on Tuesday that President Barack Obama's Financial Fraud Enforcement Task Force, which brought the latest lawsuit against Bank of America, "will continue to take an aggressive approach to combatting financial fraud and uncovering abuses in the residential mortgage-backed securities market," and is pursuing "a range of additional investigations."

Whether future investigations will succeed remains to be seen.

"Is this the first shot across the bow in terms of a larger campaign or is it trying to satisfy the press that the federal government is awake at their station but really only taking aim at a very small piece of a very big problem?" Hawthorne said.