* Congressional hearing set for Oct. 10
* Clinton sends letter pledging cooperation
* Retired ambassador Pickering to head probe panel
* No details of security requests provided
By Susan Cornwell and Tabassum Zakaria
WASHINGTON, Oct 2 (Reuters) - U.S. officials in Washington denied repeated requests from Americans in Libya for more security at the U.S. mission in Benghazi before last month's attack that killed four Americans there, two Republican lawmakers said on Tuesday.
U.S. Representatives Darrell Issa and Jason Chaffetz wrote a letter to Secretary of State Hillary Clinton demanding details of the requests for more security - which they said were made amid numerous attacks on Westerners in Libya in recent months.
They said the House of Representatives Oversight and Government Reform Committee will hold an Oct. 10 hearing on the security situation leading up to the Benghazi attack on Sept. 11.
Clinton responded later on Tuesday with a letter to the lawmakers saying the State Department would collaborate with the committee.
Issa heads the panel and Chaffetz oversees its subcommittee on national security, homeland defense, and foreign operations.
"Multiple U.S. federal government officials have confirmed to the committee that, prior to the September 11 attack, the U.S. 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," the Republican lawmakers said. Their letter did not include details of the reported requests.
"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," Clinton said in her letter. "I share that commitment."
She did not address any of the lawmakers' claims in her response.
Clinton said the State Department had named an "accountability review board" to investigate the Benghazi attack, headed by retired U.S. diplomat Thomas Pickering.
She said the panel would share its findings with lawmakers and urged them to hold off on drawing conclusions until they had seen the State Department's report.
She said the board would start its work this week and other members would include Michael Mullen, former chairman of the U.S. Joint Chiefs of Staff.
Separately, four U.S. officials have told Reuters they were aware that in the months before the Benghazi attack, some U.S. personnel in Libya had sent complaints to the State Department expressing concern about security at U.S. diplomatic facilities in Benghazi, particularly the compound where Ambassador Christopher Stevens was killed.
Two of those officials said their understanding was that the department did not act on the complaints before the attack in Benghazi.
Debate over whether the Obama administration was caught unprepared by an assault by militant groups has become U.S. election-year fodder.
Republicans have criticized initial statements by administration officials, including U.S. Ambassador to the United Nations Susan Rice, who suggested the attacks were precipitated by anger over an anti-Muslim online video.
Last Friday the top U.S. intelligence authority declared it believed this was a "deliberate and organized terrorist attack."
Stevens died of smoke inhalation when he was trapped alone inside the burning building in Benghazi, Libya's second city and the seat of last year's Feb. 17 revolt against Muammar Gaddafi.
Another diplomat, Sean Smith, also died at the compound. Two U.S. security personnel were killed later when another U.S. diplomatic compound to which some personnel retreated came under mortar attack.
Issa and Chaffetz said the violence was the "latest in a long line of attacks on Western diplomats and officials in Libya in the months leading up to" the assault.
Unarmed Libyan guards employed at the U.S. Benghazi mission were warned by their family members to quit their jobs in the weeks before the assault, "because there were rumors in the community of an impending attack," they said.
Back in April, two Libyans who had been fired from a contractor providing unarmed security for the Benghazi mission threw a small homemade bomb over the mission's fence, the letter said. No one was hurt and the suspects were arrested but not prosecuted.
Stevens had also faced threats in Tripoli, according to the letter.
It said he often took an early-morning run around the Libyan capital with his security detail, but that in June, "a posting on a pro-Gaddafi Facebook page trumpeted these runs and directed a threat against Ambassador Stevens along with a stock photo of him."
Stevens stopped the runs for about a week, but then resumed them, the letter added.