"We must do better in keeping dangerous people off airplanes, while still facilitating air travel," Obama said.
But the president acknowledged that there is no "foolproof solution" to prevent future attacks.
"As we develop new screening technologies and procedures, our adversaries will seek new ways to evade them, as was shown by the Christmas attack," he said. "In the never-ending race to protect our country, we have to stay one step ahead of a nimble adversary."
Obama's news conference had been twice delayed Thursday as intelligence officials asked for more time to "scrub" the review to make sure the unclassified version was acceptable for public release, according to sources.
Obama was first scheduled to speak at 1 p.m., then 3 p.m. and finally spoke just after 4:30 p.m. ET at the White House.
Several intelligence officials reached by ABC News conceded the missed clues but said the information received in the field was not believed to be specific or urgent enough to merit high priority at the time.
In a statement, CIA spokesman George Little defended his agency in the face of the report's conclusions, saying that before the Detroit incident, "the CIA collected and shared information about Abdulmutallab with other agencies."
On Jan. 5, CIA Director Leon Panetta instructed the agency to implement a series of reforms immediately, including new procedures to disseminate intelligence on terror suspects, expansion of name traces, and greater focus on Yemen and Africa.
The White House had indicated today's release of the so-called after-action report on the Dec. 25 incident would close the door on the investigation but that the overall review of security and intelligence procedures and structures would continue.
White House press secretary Robert Gibbs said Wednesday that the president will want to "continue to look at whether or not the progress on what has been identified and what will change [takes place and] whether we're making progress in meeting those necessary changes."
Republicans have blasted the administration for its handling of the incident.
Former vice president Dick Cheney has said Obama is "trying to pretend" the United States is not "at war."
"I think in the last three days he has [done enough]," Republican strategist Nicole Wallace said of Obama on "Good Morning America."
"[But] in the early hours after the failed terror attack, he took about six more rounds of golf and a movie with the kids," she added. "I think that first impression was that he was slow, that his surrogates were out there saying the system worked, and I think those things do irreparable harm."
Former 9/11 commission chairman Lee Hamilton told ABC News Wednesday that while the president does bear the burden of responsibility for preventing such attacks, the problem of lax vigilance throughout the government is primarily to blame.