The former commissioners, who wrote the 2004 analysis of what went wrong before and after the hijacked plane attacks, criticised anti-terrorism efforts in a range of areas from emergency communications and disaster response to keeping weapons of mass destruction out of militants' hands.
"We believe that the terrorists will strike again. So does every responsible expert that we have talked to," Thomas Kean, chairman of the commission, said at a news conference on Monday.
"We are safer but we are not yet safe. Four years after 9/11, we are not as safe as we could be. And that is unacceptable," said Kean, a Republican and former governor of New Jersey. "While the terrorists are learning and adapting, our government is still moving at a crawl."
The former commissioners issued a "report card" reviewing how the commission's 41 recommendations have been implemented and gave the government five failing grades of F, including one for not providing adequate emergency communications.
They noted that police, firefighters and other emergency workers still did not have a dedicated radio spectrum and could not communicate with each other if disaster struck.
The government earned 12 barely passing grades of D and nine mid-level grades of C. It received two "incompletes" and only one top grade, an A-minus in counter-terrorist financing.
"When will our government wake up to this challenge? Al Qaeda is quickly changing and we are not"
9/11 Commission member
Monday's report ends the work of the commissioners, which began as part of the government-appointed September 11 Commission in late 2002 and continued as the privately funded 9/11 Public Discourse Project after the commission's best-selling report was issued in 2004.
White House spokesman Scott McClellan defended the administration's security record, saying: "We have taken significant steps to better protect the American people at home. There is more to do. This is the president's highest responsibility."
The former commissioners - five Republicans and five Democrats - became the public's de facto watchdog over government efforts to fight terrorism. Now, they said, it was time for the public and elected officials to take charge.
Commission member Timothy Roemer, a former Democratic congressman, expressed concern the administration and Congress were not up to the task.
"When will our government wake up to this challenge? Al-Qaida is quickly changing and we are not," Roemer said. "We are skating on thin ice. That ice is getting thinner and about to crack."
Kean said it was scandalous that airline passengers were not fully screened against watch lists and that homeland security funding was assigned according to political priorities rather than risk.
McClellan said more work "needs to be done there to make sure that the funding is prioritised and the resources are dedicated to the greatest risks. And that's something we will continue to do".
The commission's vice-chairman, former Democratic congressman Lee Hamilton, said controlling weapons of mass destruction had to be the top national security priority.
The report faults some of Bush's
"Given the potential for catastrophic destruction, our current efforts fall far short of what we need to do," he said.
The original September 11 Commission had called on the government to strengthen counter-proliferation efforts and do more to keep weapons and highly dangerous materials scattered in Russia and other former Soviet states away from "terrorists".
"I think we've too quickly forgotten the lesson of 9/11, and I think the odds are very good that we're going to pay a terrible price for forgetting that lesson," said Republican former commissioner James Thompson, a former Illinois governor.
A statement from the Voice of September 11th, a group of victims' families, said its members were especially concerned that the sense of urgency after the 2001 attacks had dissipated, despite the former commissioners' efforts "to spur our leaders to complete the job of truly making America
House Democratic Leader Nancy Pelosi of California called the report an indictment of US counterterrorism efforts.