The White House sent a letter on Friday to House Speaker J. Dennis Hastert saying that it will now support the formation of an independent commission to investigate intelligence failures that contributed to the Sept. 11 terrorist attacks.

Until now, the White House had opposed such a commission, saying the Joint House and Senate Intelligence Committee should be able to investigate definitively what went wrong in the intelligence community. It added that a commission would be duplicative, would divert resources from intelligence matters and contribute to news leaks.

However, the White House said now that the primary focus of restructuring government and preventing future attacks is underway, it supports the creation of a commission.

"Now that the work of the intelligence committees is nearing its end, we must take the appropriate next steps," the letter said.

Response from Democrats who have supported the establishment of a bipartisan commission was quick.

"I am encouraged that the White House today ended its opposition to an independent commission to investigate all aspects of the Sept. 11th terror attacks," said House Minority Leader Dick Gephardt, D-Mo. "As I have been saying for months, we need a commission that can build on the good work of the congressional intelligence committees' joint inquiry, and help us all understand what happened, why it happened, so we can dramatically strengthen all aspects of our nation's homeland defenses."

Some members of the joint committee have said that they don't think they can do enough to uncover the details about intelligence failures because the agencies are withholding information from congressmen. That is one factor that has led to increasing support by committee members for an independent commission to pursue a more thorough investigation.

However, lawmakers were able to learn during their investigation that less than two weeks before the Sept. 11 attacks, an FBI agent given information about a potential terrorist in the United States was denied the opportunity to pursue one of the men who would later become a Sept. 11 hijacker.

The agent, who appeared Friday before the joint committee, said he was told by headquarters that the information that he obtained could not be used legally for a domestic criminal investigation.

Frustrated by the denial, the agent, wrote an e-mail to his superiors.

"I wrote on August 29, 2001, 'Whatever has happened to this, someday, someone will die. Wall or not, the public will not understand why we were not more effective in throwing every resource we had at certain problems. Let's hope the National Security Law Unit will stand behind their decision then, especially since the biggest threat to us now -- [Usama bin Laden] -- is getting the most protection,'" said the agent, whose identity was disguised at the hearing.

"I was told that everyone at headquarters was frustrated with this issues, these were the rules, NSLU does not make them up," he continued.

The agent's testimony came on day three of public hearings by the committee outlining the numerous warnings, dating back into the mid-1990's, about a plot to attack America.  The committee has been meeting since June in closed-door sessions, and is set to wrap up its investigation, but not before continuing hearings next week into other intelligence blunders.

While those hearings continue, some lawmakers have expressed concerns that public disclosures of what went wrong could serve to tell terrorists some of the U.S.'s sources and methods of counter-terrorism, and it could help them obstruct investigations in the future.

In the meantime, lawmakers took action last year to help prevent a repeat of the agent's assertion of a failure to operate. Congress overwhelmingly passed last year's USA PATRIOT Act, which makes it easier to pursue terrorist investigations that begin overseas and lead into the country. Other regulations and rules used by the FBI and Department of Justice have also changed.

The agent's testimony is one of many examples of missed opportunities to pursue the Sept. 11 hijackers, according to a congressional investigator.

Eleanor Hill, staff director for the Sept. 11 inquiry on Capitol Hill, said two of the Sept. 11 hijackers, Nawaf al-Hazmi and Khalid al-Mihdhar, had been spotted at an Al Qaeda meeting in Malaysia 18 months before the attacks. In March 2000, a CIA station informed the FBI that al-Hazmi had flown into Los Angeles on Jan. 15, 2000, just after the Malaysia meeting. The CIA agent did not request that any action be taken.

Intelligence agencies "had, but missed, opportunities both to deny them entry into the United States and subsequently to generate investigative and surveillance action regarding their activities within the United States," Hill said.

The two terrorists, who were aboard American Airlines Flight 77 that crashed into the Pentagon, lived openly under their real numbers in San Diego, rented an apartment, one obtained a driver's  license and both took flight lessons.

U.S. authorities also had some information about Salim-Al-Hazmi, brother of Nawaf, and also on flight 77. Officials had no information on the other 16 hijackers, according to Hill's investigation.

The two men were not put on the State Department's watch list for denying visas until Aug. 23, 2001. Hill said CIA officials told her there were no procedures at the CIA Counterterrorism Center for putting suspects on watch lists and they had received no training on watch lists.

On Thursday, administration officials said they did not have enough information to know when and where an attack using airplanes as missiles would take place, but that had the State Department been notified of the two suspected terrorists, it likely would have denied them entry into the country.

The chairman of the House Intelligence Committee, Rep. Porter Goss, R-Fla., told reporters Thursday that he was reserving judgment until receiving the report about whether the failure to stop al-Mihdhar and al-Hazmi was a clear-cut intelligence blunder.

"From what I have seen so far, there are questions that need to be asked to determine if there were mistakes," said Goss, a former CIA officer.

Fox News' Carl Cameron and the Associated Press contributed to this report.