US officials in Libya repeatedly saw their requests for extra security for the mission in Benghazi refused by Washington before the attack that killed the US ambassador, according to two Republican politicians.
US Representatives Darrell Issa and Jason Chaffetz wrote to Secretary of State Hillary Clinton to demand details of the requests, which they said were made after a number of attacks on Westerners in Libya.
"Multiple US federal government officials have confirmed to the committee that, prior to the September 11 attack, the US mission in Libya made repeated requests for increased security in Benghazi," Issa and Chaffetz wrote.
"The mission in Libya, however, was denied these resources by officials in Washington."
Ambassador Christopher Stevens died of smoke inhalation after being trapped in a burning building. Another diplomat, Sean Smith, also died at the compound.
Two US security personnel were killed later when an area some diplomatic staff had fled to came under mortar fire.
The Obama administration initially suggested the violence was sparked by anger over an anti-Muslim film, produced in the US, which had been posted on the internet.
But last week, the senior US intelligence authority said it was a "deliberate and organised terror attack".
According to Issa and Chaffetz, Libyan guards employed at the Benghazi mission were warned by family members to quit their jobs in the weeks before the assault, "because there were rumours in the community of an impending attack".
They also said that early-morning runs the ambassador took with his security detail had been "trumpeted" on a pro-Gaddafi Facebook page. The page featured a picture of Mr Stevens and a threat against him.
The House of Representatives Oversight and Government Reform Committee will hold a hearing into the security situation at the Benghazi mission. Issa is the head of the panel and Chaffetz oversees its sub-committee on national security, homeland defence, and foreign operations.
Mrs Clinton has said she will collaborate with the committee.
"I appreciate that you and your committee are deeply interested in finding out what happened leading up to and during the attacks in Benghazi, and are looking for ways to prevent it from happening again," she said.
FBI agents were sent to Libya in the aftermath of the attack but are still to reach an agreement with the Libyans on how the investigation will be conducted and have not even visited Benghazi.
Libyan officials say eight people have been arrested so far in connection with the attack.
Meanwhile, it has been reported that small teams of special forces troops were sent to US embassies across North Africa to set up a counter-terrorism network in the months before the Benghazi attack.
Unnamed officials said the network, which was approved a year ago, was too recent to stop the attacks.