The U.S. government suggested al-Qaida elements may have played a role in a Sept. 11 attack in Benghazi that left the U.S. ambassador and members of his staff dead. The International Committee of the Red Cross had pulled its staff out of Benghazi weeks before the attack and a travel advisory from the State Department was in effect.
The letter, published in part by Washington publication The Hill, states that U.S. diplomats in Libya had made requests for increased security in Benghazi before the attack.
"The mission in Libya, however, was denied these resources by officials in Washington," the letter stated.
Clinton had called on the FBI to conduct an investigation into the attack.
Victoria Nuland, a spokeswoman for the State Department, offered few specifics because of the ongoing investigation, however.
"We want to get to the bottom of precisely what happened and learn any lessons that we need to learn from it," she said. "We're taking this very, very seriously."
A congressional hearing on the attack is scheduled for next week.