WASHINGTON - Congressional Republicans on Thursday challenged the Obama administration's plan for correcting flaws exposed by the September attacks on the U.S. mission in Libya, pressing for changes to its approach to security and asking what Secretary of State Hillary Rodham Clinton and other top officials knew before the deadly assault.

Three days after an independent investigative panel delivered a stinging report on the department's failures in Benghazi, Republican lawmakers in open hearings demanded to know why State Department officials did not do more to protect the mission when they were clearly aware that attacks on Western targets had increased all year.

The Republicans didn't appear to open any new areas for investigation, however. And with a number of GOP lawmakers not showing up for the hearings, it appeared that an issue that has been a major focus of conservatives' efforts since the fall may be losing steam.

The Sept. 11 attacks killed U.S. Ambassador J. Christopher Stevens and three other Americans, and set in motion a broad reexamination of how the State Department protects its 275 posts around the world.

Sen. Bob Corker, R-Tenn., asked Clinton's deputies, William J. Burns and Thomas R. Nides, why the secretary hadn't sought to shift money to better protect the Benghazi mission, given the stream of violent incidents in the city and warnings from lower-level U.S. officials.

"Why did [Clinton] never ask for ... any change of resources to make sure Benghazi was secure? Why did that not happen?" Corker said at a hearing of the Senate Foreign Relations Committee. "I cannot imagine that we had people out there with the lack of security. ... What I saw in the report is a department that has sclerosis."

Administration officials accepted the conclusion of the investigative panel that security arrangements were deeply flawed, and they have sought to show they are taking the initiative on the issue.

State Department officials have embraced all 29 recommendations from the Accountability Review Board and have removed from their jobs -- but not the department -- four officials with some responsibility for diplomatic security in Libya. They are asking Congress for more money and reallocating funds to pay for more Marine guards, civilian security personnel and physical security improvements.

Sen. Marco Rubio, R-Fla., said the Accountability Review Board's report "places a lot of the blame on lower-level officials" and asked how aware Clinton and her top aides were of the concerns about the threat in Libya.

Burns said there were memos circulating among Clinton and top aides discussing the "deteriorating security situation in Libya." But he said he was not aware of any paper flow to the top level that discussed lower-level officials' requests for more money for security in Benghazi.

The Republicans rejected arguments by administration officials and their Democratic supporters that part of the problem was the tight budget for diplomatic security, which House Republicans reduced last year. They called instead for a shift of money from lower-priority items.

At an afternoon hearing, the chair of the House Foreign Affairs Committee, Rep. Ileana Ros-Lehtinen, R-Fla., said the problem was "misplaced priorities." She said the department was "lavishing" money on projects such as a "culinary diplomacy partnership" that sends American chefs around the world, and efforts to slow climate change.

Clinton asked on Saturday to be excused from appearing because she had fallen and suffered a concussion earlier last week. She has agreed to testify in mid-January, Ros-Lehtinen said.

Meanwhile, at least four people were killed after militants attacked a security headquarters in Benghazi Thursday night. The attack on the security directorate building -- with rocket-propelled grenades and hand grenades, according to authorities and witnesses -- was the latest in a string of attacks on police facilities since last month, when unknown assassins killed Benghazi's security director outside his home.

The attack Thursday appeared to be an attempt by militants to free suspects held in connection with the death of Faraj Mohammed al-Drissi, the security director, and other assassinations.

The State Department said Wednesday night that three officials who had been criticized by the investigative board are giving up their jobs, and Eric Boswell, head of the Diplomatic Security bureau, agreed to resign his post. The three others have been "placed on administrative leave pending further action," a department statement said.

The statement didn't identify the three, but other officials said two of them are Charlene Lamb, a deputy assistant secretary in the Diplomatic Security bureau, and Raymond Maxwell, a deputy assistant secretary in the Near East Affairs bureau.

The New York Times contributed to this report.