WASHINGTON — Despite two explosions and dozens of other security threats, U.S. officials in Washington turned down repeated pleas from American diplomats in Libya to increase security at the U.S. Consulate in Benghazi where the U.S. ambassador was killed, Republican leaders of a House committee said Tuesday.

In a letter to Secretary of State Hillary Rodham Clinton, Chairman Darrell Issa and Rep. Jason Chaffetz of the Oversight and Government Reform Committee said their information came from “individuals with direct knowledge of events in Libya.”

Issa, R-Calif., and Chaffetz, R-Utah, said the attack three weeks ago in Benghazi that killed Ambassador Chris Stevens and three other Americans was the latest in a long line of attacks on Western diplomats and officials in Libya in the months before the 11th anniversary of the Sept. 11 terror attacks on the U.S.

The letter listed 13 incidents, but Chaffetz said in an interview there were more than 50. Two of them involved explosive devices: a June 6 blast that blew a hole in the security perimeter. The explosion was described to the committee as “big enough for 40 men to go through”; and an April 6 incident where two Libyans who were fired by a security contactor threw a small explosive device over the consulate fence.

“A number of people felt helpless in pushing back” against the decision not to increase security and “were pleading with them to reconsider,” Chaffetz said. He added that frustrated whistle-blowers were so upset with the decision that they were anxious to speak with the committee.

The lawmakers said they plan a hearing on Oct. 10. They asked Clinton whether the State Department was aware of the previous incidents, and whether the level of security that was provided to the U.S. mission met the security threat, and how the department responded to requests for more protection.

The State Department has declined to answer questions about whether extra security was sought by officials in Benghazi ahead of the attack.

Clinton responded in a letter to Issa on Tuesday that she has established an accountability review board that will determine “whether our security systems and procedures in Benghazi were adequate, whether those systems and procedures were properly implemented, and any lessons learned that may be relevant to our work around the world.”

She asked the committee to withhold any final conclusions about the Benghazi attack until the committee can review the findings of the board. The panel is led by retired State Department official Thomas Pickering and includes retired Adm. Mike Mullen, former chairman of the Joint Chiefs of Staff.

Clinton also pledged to address the specific questions raised in the committee’s letter in addition to document requests.

Referring to the Benghazi attack, the letter said, “It was clearly never, as administration officials once insisted, the result of a popular protest.”

In the days immediately after the attack, the Obama administration said it believed it was a spontaneous reaction to an anti-Islamic video circulating on the Internet. Since then, Defense Secretary Leon Panetta and White House press secretary Jay Carney have called the incident a terrorist attack. President Barack Obama has not used those precise words, though he has referred to the attack in the context of “acts of terror.”

Republicans have lashed out at Obama and senior administration officials over their evolving description of the deadly Sept. 11 attack, a late campaign-season broadside.

Republicans sensed a political opportunity in Obama’s apparent reluctance to utter the words “terrorist attack” as well as the varying explanations emerging from the administration about the assault in Benghazi.

Republican presidential candidate Mitt Romney told Fox News recently: “I think it’s pretty clear that they haven’t wanted to level with the American people. We expect candor from the president and transparency.”

U.S. and Libyan investigators still aren’t working together three weeks after the attack. State Department spokeswoman Victoria Nuland said Tuesday the U.S. wants maximum transparency, collaboration and cooperation. She said the U.S. and Libya were partnering well politically, but there now has to be cooperation at the investigative level.

Clinton discussed security on Sept. 18, when asked whether measures were appropriate.

“Let me assure you that our security in Benghazi included a unit of host government security forces, as well as a local guard force of the kind that we rely on in many places around the world,” she said.

“In addition to the security outside the compound, we relied on a wall and a robust security presence inside the compound,” she said. “And with all of our missions overseas, in advance of Sept. 11, as is done every year, we did an evaluation on threat streams.”

Clinton also said the Office of the Director of National Intelligence “has said we had no actionable intelligence that an attack on our post in Benghazi was planned or imminent.”

She added that diplomats “engage in dangerous work, and it’s the nature of diplomacy in fragile societies and conflict zones to be aware of the necessity for security but to also continue the important diplomatic work that has to go on.

“There is risk inherent in what we do and what these brave men and women representing the United States are up against every single day,” Clinton said, “and we do our very best to limit that risk by ensuring that our security protocols reflect the environments in which diplomats work and the threats that they are presented with.”

Chaffetz said in an interview that if the Benghazi security was typical of the protection in other dangerous places, “that’s frightening.” He said Benghazi is “one of the most awful and volatile areas on the planet. A reduction in the security profile doesn’t seem consistent with the threat that was on the ground. That’s why we have whistleblowers anxious to speak with us.”

Among the incidents cited in the Issa-Chaffetz letter to Clinton:

—Just weeks before the attacks, the unarmed Libyan guards at the consulate, employed by British contractor Blue Mountain Group, were warned by family members to quit their jobs because there were rumors of an impending attack.

—In April, a gun battle erupted about two miles from the consulate between an unidentified armed group and forces loyal to the transitional government.

—In June, a posting on a Facebook page mentioned Stevens’ early morning runs around Tripoli along with members of his security detail. The page contained a threat against Stevens and a stock photo of him. Stevens stopped the runs for about a week, but then resumed.