WASHINGTON (CNN) -- The FBI is investigating Fannie Mae, Freddie Mac, Lehman Brothers and AIG - and their executives - as part of a broad look into possible mortgage fraud, sources with knowledge of the investigation told CNN Tuesday.

Two officials with knowledge of the FBI investigation into the mortgage crisis said "the investigation is all very preliminary". They said there is a lot of anger and people want someone held accountable.

Officials are looking into whether any criminal activity occurred, but the Bureau said the investigation will take some time. They said the investigation is in the preliminary stages, and so far it is a broad look at the companies involved.

"From what I've seen so far, I really don't believe we're going to find widespread fraud," according to one of the officials. They said they have to go where evidence and facts lead. Just because an investigation has been opened doesn't mean there will be charges.

At this point, the officials said they are not pursuing specific individuals at the four firms under investigation because these firms are facing serious financial problems and they need to find out if it's the result of criminal activity.

Again, the officials stressed this is going to be a long road. "Don't expect indictments tomorrow or next week or next month," one of the officials told CNN.

FBI spokesman Special Agent Richard Kolko had no comment on that information, but said 26 firms are currently under investigation as part of the bureau's mortgage fraud inquiry.

Earlier this month, FBI Director Robert Mueller told Congress that 1,400 individual real estate lenders, brokers and appraisers are now under investigation in addition to two dozen corporations.

"The FBI currently has 26 pending corporate fraud investigations involving subprime lenders," Kolko said. "As we have seen, this number can fluctuate over time; however we do not discuss which companies may or may not be the subject of an investigation."

Previously, CNN has reported that lender Countrywide and Atlanta-based homebuilder Beazer (BZH) - which dropped out of the mortgage business early this year - are part of the investigation.

The sources said the probes of Fannie (FNM, Fortune 500), Freddie (FRE, Fortune 500), Lehman (LEH, Fortune 500) and AIG (AIG, Fortune 500) are believed to be in the early stages. One source said the government would be "remiss" if it didn't look into what happened at these companies because of the financial problems they are involved in and the actions of individuals running them.

Lehman Brothers and Freddie Mac declined comment on the matter. AIG spokesman Joseph Norton said, "We don't have details about the FBI investigation. Of course, we will cooperate with the FBI."

The United States is in the midst of a spiraling economic crisis fueled largely by the housing market. Earlier this decade, mortgage lenders relaxed restrictions on obtaining mortgages as home prices soared about 85% from 1996 through 2006 in inflation-adjusted dollars, creating a bubble. Then the bubble popped, and lenders - as well as mortgages - took the hit.

Last week, mortgage insurer AIG narrowly avoided bankruptcy when the federal government took 80% of its equity in exchange for an $85 billion loan from the Federal Reserve while Lehman filed the largest bankruptcy in American history. Earlier this month, the government took over mortgage giants Fannie and Freddie.

Bank of America (BAC, Fortune 500) bought Countrywide in July. Beazer dropped its mortgage arm early this year after an internal investigation - prompted by a Charlotte Observer investigation - found "evidence that employees violated [federal] regulations ... back to at least 2000." The company said it is cooperating with the federal investigation.

Other bank failures and takeovers have led to the Bush administration's current proposal to spend $700 billion to shore up the financial markets. The proposal is under consideration by Congress, where lawmakers from both sides of the aisle have balked at the proposal's lack of oversight provisions, among other issues.

As the mortgage industry began to unravel, the FBI, with assistance from the IRS, launched a broad investigation into mortgage fraud. In June, its Mortgage Fraud Task Force arrested more than 400 mortgage brokers, lenders, appraisers and other industry insiders who, it said, were responsible for more than $1 billion in losses.

Last month, a Mortgage Asset Research Institute (MARI) study found that the number of fraudulent loans issued during the first three months of 2008 skyrocketed 42% compared with the same period in 2007.