(Reuters) - A U.S. Justice Department inquiry into the packaging and sale of home loans by the biggest U.S. banks casts a wide net and appears to significantly overlap with other enforcement efforts, according to people who have viewed subpoenas sent to the firms.

The civil subpoenas that were sent in January ask for documents related to every offering between 2006 and 2008, including bonds backed by Fannie Mae and Freddie Mac, three people familiar with the matter said.

An older investigation by the U.S. Securities and Exchange Commission focused on the first two years, and limited its scope to private offerings, the people said.

The general nature of the subpoenas and the overlap with SEC inquiries suggest investigations related to the financial crisis could drag on for years.

The Justice Department announced in January that it had sent civil subpoenas to 11 financial institutions about the market for residential mortgage-backed securities.

The probe is part of a new inter-agency task force that President Barack Obama unveiled in his State of the Union speech. It is designed to add firepower to investigations into the sale of repackaged mortgages that helped fuel the housing bubble and spread around exposure to toxic loans.

The breadth of the new subpoenas, which also overlap with previous requests from the Justice Department, is leaving some lawyers who are representing banks in the requests befuddled about just want the government is after.

Citigroup disclosed last week that it had received one of the Justice Department subpoenas, and sources familiar with the inquiry said it is aimed at the largest U.S. banks.

"This is a difficult environment to figure out where this is headed; there are a lot of overlapping cooks in the kitchen," said one source, who declined to be named.

"The government just keeps asking for the same kind of information with a different twist," another said, "it's going to be a full-employment-act for lawyers."

Justice Department spokeswoman Adora Andy said the working group is marshaling parallel efforts to collaborate on current and future investigations, pooling resources and streamlining processes to investigate those responsible for misconduct.

The SEC did not immediately respond to a request for comment on their investigations.

In a sign of the uneven advancement of enforcement efforts, three banks earlier this week disclosed they may face SEC charges related to mortgage-backed securities that soured in the financial crisis.

JPMorgan Chase & Co, Goldman Sachs Group Inc, and Wells Fargo & Co all disclosed they had received Wells notices from the SEC, a document that alerts defendants that SEC lawyers are considering bringing charges and gives them a chance to rebut the allegations.

The notices signal the SEC cases are advanced and essentially mark the start of settlement negotiations.

New Justice Department subpoenas could gum up efforts to resolve any SEC case, since companies dislike resolving an enforcement action from one agency if related charges from another are still likely.

"You may see firms fighting more, because they can't just settle one and have all the others out there," one of the lawyers said.

When Attorney General Eric Holder announced the subpoenas at a January news conference to promote the inter-agency task force, he said the new requests do not duplicate earlier efforts from the SEC.

At the same news conference, SEC enforcement director Robert Khuzami said his agency had already reviewed 25 million pages of documents on related investigations into residential mortgage-backed securities.

"To be clear, investigations into RMBS offerings have been ongoing at the SEC," Khuzami said at the time.

(Reporting by Aruna Viswanatha; Editing by Tim Dobbyn)