WASHINGTON — Despite two explosions and dozens of other security threats, US officials in Washington turned down repeated pleas from American diplomats in Libya to increase security at the US Consulate in Benghazi where the US 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 Representative Jason Chaffetz of the Oversight and Government Reform Committee said their information came from ‘‘individuals with direct knowledge of events in Libya.’’

Issa, of California, and Chaffetz, of 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 United States.

The letter listed 13 specific incidents, but Chaffetz said in an interview there were more than 50. Two of them involved explosive devices. The first was 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.” The second was an April 6 incident where two Libyans who had been fired by a security contractor 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 US 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.’’