(CBS News) WASHINGTON - On Wednesday, an independent investigation blistered the State Department for not providing adequate security before the U.S. ambassador to Libya and three other Americans were killed in September.
On Thursday, the State Department tried to explain itself before Congress.
Secretary of State Hillary Clinton's top deputies told the Senate Foreign Relations Committee, which oversees the department, that changes in security are already underway.
Deputy Secretary Tom Nides, who is in charge of the overhaul, said: "We've got to learn from this. We've got to hold people accountable, which we're doing, and we have to change processes to make sure we're getting it right."
Changes include: A new high threat unit to oversee posts in 19 dangerous locations like Libya; the addition of more than 150 security agents and 225 more Marines to defend those posts; and the exit of the department's top four security officials.
Full coverage: The U.S. Consulate attack in Benghazi
The Benghazi report noted that chronic budget pressure kept managers from spending money on needed security. In response, Secretary Clinton is asking Congress for $1.4 billion above the initial budget request for next year.
Senator John Kerry, who chaired the hearing, said: "Congress also bears some responsibility here. Congress has the power of the purse. We use it for any number of things, but it's our responsibility."
Kerry is the frontrunner to replace Clinton as secretary of state when she steps down next month.
Republican Senator Bob Corker said that for him, it's not just a money issue: "I'm just saying that the culture within the State Department, to me, is one that needs to be transformed. This committee can help, maybe the next secretary of state can help, but the fact is there's a lot of work that needs to be done there."
Lawmakers confirmed that Secretary Clinton, who is at home recovering from a concussion, will testify in January. They also learned that the FBI investigation into which terrorists carried out the attack on the U.S. post is ongoing.