The FBI is investigating whether Countrywide, America’s biggest mortgage lender, lied to shareholders and regulators about the quality of its loan book and finances, amid concern that the credit crunch may be entering a new and dangerous phase.

The FBI’s move comes as part of an inquiry into the way in which 15 lenders behaved leading up to and during America’s sub-prime mortgage crisis, which has rendered billions of dollars of bonds effectively worthless.

The investigation into the veracity of financial statements made to the US Securities and Exchange Commission, Wall Street’s regulator, by the mortgage lender has also raised questions about whether Bank of America, which is in the process of buying Countrywide for $4 billion (£2 billion), is acquiring the group at an inflated price. The deal had been expected to be completed in August this year.

Although the FBI declined to comment, it is believed that its inquiry is in the preliminary stages. It is believed that the FBI is trying to find out whether Countrywide is hiding losses far bigger than it has already accounted for – the lender is estimated to have issued more than $100 billion of mortgage-backed securities between 2004 and 2007. It is already under investigation by the SEC for any accounting irregularities.

The FBI has been trying to ascertain whether 15 lenders have been fraudulent with mortgages they sold, whether they harboured undisclosed conflicts of interest and how they packaged and sold on the mortgages to Wall Street.

The inquiry is the latest development in the credit crisis that erupted on Wall Street last summer, which has spilled over into the wider economy and is expected to plunge America into recession. The Federal Reserve is expected to cut interest rates again when it meets on March 18, by half a percentage point to 2.5 per cent.

Last summer defaults on mortgage repayments soared as consecutive interest rate rises triggered a surge in foreclosures. Those mortgages - some of which were sold to people with patchy credit histories – were pooled together and sold on to banks, hedge funds and fund managers. Much of that sub-prime debt isnow untradeable.

At the end of January, Neil Power, the head of the FBI’s economic crimes unit, announced the launched of a criminal investigation into the mortgage industry, to examine every level of the conspiracy that it believes perpetuated the housing boom, and ultimately resulted in millions of Americans losing their houses and investment banks losing billions of dollars.

At the time, Mr Power said: “We’re looking at the accounting fraud that goes through the securitisation of these loans. We’re dealing with the people who securitise them and then the people who hold them, such as the investment banks.” He said that he was also concerned that some banking executives might be guilty of insider-trading, offloading collateralised debt obligations, pools of bonds and other securities backed by mortgages before their true valuations came to light in the wake of the home loan meltdown.

The FBI suspects that the house-price boom encouraged mortgage lenders to take bigger risks, making loans to people with weaker and weaker credit histories as they sought new clients. It is investigating whether these lenders, and the brokers that arranged the mortgages, encouraged borrowers to lie about their income.

Countrywide and Bank of America did not return calls yesterday.

Analysts and bankers fear that the credit crunch may be entering a new, more damaging phase as highly leveraged structured firms and hedge funds - even those with blue chip mortgage investments – struggle to meet margin calls from their banks.

Adding to the credit market jitters has been the soaring cost of insuring against credit default by all kinds of borrowers. Falling asset prices can trigger forced fire sales of securities in leveraged specialist vehicles, adding to the risk of a vicious circle of falling prices and liquidations.

Over the weekend Carlyle Group was debating what to do about its Amsterdam-listed investment company, Carlyle Capital Corporation (CCC), which has failed to meet margin calls and has received default notices from banks owed about $21 billion. CCC shares were suspended on Friday amid fears that it might go into liquidation unless Carlyle injects fresh equity or guarantees new credit lines. A progress statement is expected from CCC today.

CCC’s huge portfolio is mostly invested in securities issued by blue-chip mortgage finance institutions, such as Fannie Mae and Freddie Mac, that are backed by prime mortgages, but even these securities have dropped in price in recent weeks, obliging lenders to seek additional collateral.