Congressional leaders have said US diplomats in Libya made repeated requests for increased security in the eastern city of Benghazi, but the pleas were denied in Washington.
The House Oversight and Government Reform Committee officials said on Tuesday they received the information from individuals with direct knowledge of the situation.
Al Jazeera's Kimberly Halkett, reporting from Washington, said the state department has been tightlipped about the matter.
"The state department has been asked about this repeatedly, and the answer has always been no ... [that] there hadn't been prior requests for additional security," she said.
"But now we are getting some new information from the House Oversight and Government Reform Committee - a Republican controlled committee [...] outlining new information that they received from multiple employees speaking in confidence to the committee," our correspondent said.
There were 13 security threats in the six months prior to the September 11 attacks, Halkett said, adding: "This is really a different picture than what we have been getting for many weeks."
The September 11 attack on the US consulate in Benghazi killed Ambassador Chris Stevens and three other Americans.
The committee said the attack was the latest in a long line of attacks on Western diplomats and officials in Libya in recent months.
The committee plans a hearing on October 10.
Darrell Issa, the committee's chairman, and Jason Chaffetz wrote to Hillary Clinton, the US secretary of state, asking for details on the state department's response to requests for additional security.
Clinton will respond to the legislators on Tuesday, said Victoria Nuland, the state department spokeswoman.
Clinton is ready to co-operate "closely" with Congress in investigating the attack in Benghazi, Nuland said at a regular daily news briefing.