‘‘For many years the State Department has been engaged in a struggle to obtain the resources necessary to carry out its work with varying degrees of success,’’ it said. This has led to efficiencies but also ‘‘had the effect of conditioning a few State Department managers to favor restricting the use of resources as a general orientation.’’

It said the number of Diplomatic Security staff in Benghazi before and on the day of the attack ‘‘was inadequate despite repeated requests ... for additional staffing.’’

‘‘The solution requires a more serious and sustained commitment from Congress to support State Department needs, which, in total, constitute a small percentage of the full national budget and that spent for national security,’’ it said. ‘‘Congress must do its part to meet this challenge and provide necessary resources.’’

Congress has denied some funding requests from the State Department for more security.

In her letter to lawmakers, Clinton urged Congress to support the department’s security requests, noting that ‘‘all of us ... have a responsibility to provide the men and women serving this country with the best possible security and support.’’

Retired Ambassador Thomas Pickering and a former Joint Chiefs of Staff chairman, Adm. Mike Mullen, led the independent review, studying thousands of pages of cables and other documents, hours of video and intelligence and interviewing more than 100 people, including survivors. They will testify before the House and Senate foreign affairs committees behind closed doors on Wednesday.

On Thursday, the State Department’s two deputy secretaries, William Burns and Thomas Nides, will testify in open sessions before the House Foreign Affairs Committee and the Senate Foreign Relations Committee.

Clinton was to have appeared at Thursday’s hearing but canceled after fainting and sustaining a concussion last week while recovering from a stomach virus that dehydrated her. Clinton is under doctors’ orders to rest.

The Benghazi attack has highlighted the larger question of how U.S. diplomats and intelligence officers can do their jobs in unstable environments, as al-Qaida spreads across Africa, without also expanding their security. Diplomats have said that overreacting to the attack could produce what some are calling a ‘‘Benghazi effect’’ that would wall them off from the people they are supposed to be engaging.

In her letter to lawmakers, Clinton said, ‘‘We will never prevent every act of terrorism or achieve perfect security’’ but she stressed that ‘‘our diplomats cannot work in bunkers.’’

‘‘We must accept a level of risk to protect this country we love and to advance our interests and values around the world,’’ she said.

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AP Intelligence Writer Kimberly Dozier contributed to this report.

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