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WASHINGTON -- President Obama signaled Thursday that he doesn't intend to fire any top officials over the attempted bombing of a Northwest Airlines flight, even as he repeated his claim that U.S. intelligence had enough information to uncover the plot but failed to connect the dots.

Following two weeks of speculation over whether the administration's internal review would result in forced resignations, the president said there's plenty of blame to go around -- but "ultimately the buck stops with me."

Obama, in unveiling a summary of his administration's initial review of the security breakdown, announced Thursday afternoon a series of policy changes intended to prevent a repeat of the kind of attack attempted on a Detroit-bound flight on Christmas Day. Obama seemed to make clear that those changes – not firings – would stand as his administration’s corrective action.

"I have repeatedly made it clear in public with the American people and in private with my national security team that I will hold my staff, our agencies and the people in them accountable when they fail to perform their responsibilities at the highest levels," Obama said. "Now, at this stage in the review process it appears that this incident was not the fault of a single individual or organization, but rather a systemic failure across organizations and agencies."

Click here to read a summary of the White House review of the failed attack.

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Several top officials have come under scrutiny since the attempted bombing. Homeland Security Secretary Janet Napolitano drew criticism for declaring "the system worked" the Sunday after the failed attack. Michael Leiter, director of the National Counterterrorism Center, went on a ski trip after the attempt, reportedly raising concerns in the intelligence community.

And in the aftermath, possible missteps at several agencies emerged. The CIA had been sitting on a report about suspect Umar Farouk Abdulmutallab. The NCTC and CIA did not search all available databases to find "derogatory information" about Abdulmutallab, according to the summary of the internal review. Other information was not pieced together that could have gotten him on the no-fly list, Obama said. The summary also said that the State Department initially thought the Abdulmutallab did not have a valid U.S. visa, because of a "misspelling" of his name – though he did have a visa.

Obama said he is "less interested in passing out blame" than learning from the attempt and "correcting these mistakes to us safer."

"As president, I have a solemn responsibility to protect our nation and our people, and when the system fails, it is my responsibility," Obama said.

Republicans welcomed the White House's decision to release its report on the failed attack, but they criticized the administration's overall strategy of fighting terrorism.

Texas Sen. John Cornyn, a member of the Judiciary Committee, said he's worried that the president is too distracted by his domestic agenda to focus on keeping the nation safe.

"I appreciate the president's admission that mistakes were made here, but this is the same conclusion the administration reached in the aftermath of the terrorist attack on Fort Hood," Cornyn said.

"Our national security adviser said that the president 'certainly doesn't want that third strike,'" he said. "In light of this, I am hopeful the administration will use this third chance to address the failures that allowed the Christmas Day attempt, as well as the Fort Hood attack, to occur -- the safety of the American people depends on it."

Obama ordered U.S. agencies to assign specific responsibilities for investigating all leads of terrorist threats so they are tackled "aggressively" in the wake of the attempted attack. And he ordered the agencies to move faster and more accurately in adding suspects to a watch list designed to stop terrorists before they strike.

But he cautioned that there is "no silver bullet" or "foolproof way" to protect Americans from terrorists.

Obama's top counterterrorism aide, John Brenann, blamed himself for the security failure that allowed a would-be bomber suspected of Al Qaeda ties to smuggle an explosive on board a plane to Detroit in his underwear.

"I told the president today I let him down," Brennan said at a press briefing following the president's remarks. "I am the president's assistant for homeland security and counterrorism and I told him I will do better and we will do better as a team."

The unclassified six-page summary of the report given to Obama stated that U.S. intelligence officials had received unspecified "discrete pieces of intelligence" to identify Abdulmutallab, a 23-year-old Nigerian, as an Al Qaeda operative and keep him off the plane. Officials received fragments of information as early as October, according to the report.

Although intelligence officials knew that an Al Qaeda operative in Yemen posed a threat to U.S. security, officials did not increase their focus on that threat and did not pull together fragments of data needed to foil the scheme, said the summary.

"The American people should not have to rely on luck and passengers' sheer determination to stop terrorist attacks," Michigan Rep. Pete Hoekstra, the top Republican on the House Intelligence Committee, said in a written statement. "They need a government that is working together on the executive and congressional levels to help keep our homeland safe."

He said the Fort Hood massacre and the latest plot show that government is failing in that mission, and he urged the White House to also publicize its report on Fort Hood.

While the administration's report on the airline plot notes problems in pursuing separate pieces of intelligence gathered before the attempted airline attack, it concludes "the watch listing system is not broken" and a reorganization of the nation's counterterrorism system is not necessary. The report, instead, calls for strengthening the process used to add suspected terrorists to watch lists.

According to the report, "a series of human errors" occurred, including a delay in the dissemination of a completed intelligence report and the failure of CIA and counterterrorism officers to search all available databases for information that could have been tied to Abdulmutallab.

Unlike the run-up to the 2001 terrorist attacks, intelligence officials shared information. But authorities didn't understand what they had.

Abdulmutallab was indicted Wednesday on charges of attempted murder and other crimes for trying to blow up an airliner.

Fox News' Mike Emanuel and The Associated Press contributed to this report.