The Social Security Administration relaxed its privacy restrictions because of the emergency nature of the investigation, according to the documents.
The memos obtained by the Electronic Privacy Information Center through the Freedom of Information Act show the Social Security Administration responded only to requests concerning the terrorist attacks.
"The commissioner of Social Security has exercised the ad hoc authority in disclosure regulations to allow the release of relevant information in response to these requests," a policy document said.
Spokesman Mark Lassiter told The Associated Press his agency could still provide information related to the September 11 attacks to law enforcement officials but that such demands have decreased.
He said the 9/11 investigation was not the first time the authority was used.
But Marcia Hofmann, staff counsel at the privacy group known as EPIC, told the AP: "This is a very broad policy that leaves a great deal of discretion in the hand of individual law enforcement agents."
The FBI Wednesday declined to discuss the information it reviewed from the Social Security Administration.
In a letter to the chairmen of two powerful House committees, Rep. Carolyn Maloney called for hearings to determine why Congress was not informed of the decision to allow FBI investigators access to the records.
Maloney, however, did not charge the action was improper.
"It seems that in the specific and extreme instance of 9/11, SSA may have acted correctly, but the lack of general knowledge about its actions leads to bigger and more troubling questions," she said.
Officials with the two committees, House Ways and Means and House Government Reform, had no immediate response on the call for hearings.
The FBI has said the September 11 hijackers were able to open several U.S. bank accounts using illegitimate Social Security numbers.