One Sunday evening last March, a baggage handler was loading up an Airbus jet when he noticed that a passenger's bag was burning.

Officials halted the flight, found the passenger, and told him they were taking all of his baggage off the plane for re-screening before the flight could continue, the captain said in a report to federal aviation officials.

In the burnt bag, authorities discovered a suspicious bundle of wires and a video game battery pack. The captain, meanwhile, thought the passenger's behavior was suspect. He had a one-way ticket, volunteered to be removed from the flight without being asked, and said he would fly another day.

"Had we left the gate on time, we would have been airborne when this bag ignited," the report noted.

The captain complained in his filing with the federal Aviation Safety Reporting System that there was no security follow-up although many questions remained: "Who was this individual? Were his actions intentional? Why was his behavior so abnormal?"

Despite an extensive security effort since the 2001 terrorist attacks, those kinds of questions are still being asked with alarming frequency at U.S. airports.

Many steps have indeed been taken, including creation of the Transportation Security Administration (TSA), to improve security at the nation's 400 commercial airports and at all airlines.

But six years later, the TSA still falls short in 7 of 24, or almost one-third, of critical performance benchmarks set for the agency, an August 2007 federal report says. The shortfalls included securing areas of airports that are supposed to be restricted and adequate screening of air cargo, according to the Government Accountability Office (GAO), the federal government's audit agency.

Some of those problems were cited in a March 2007 Consumer Reports investigation, An accident waiting to happen?, which found that increased outsourcing by airlines had created safety and security loopholes. Now, even for some areas in which the TSA has supposedly met its goals, Consumer Reports has found major security lapses, including the following:

Screening failures. The TSA has an erratic record at checkpoint screening, including failures during undercover tests to identify weapons and explosives. A November 2007 GAO report found that agents smuggled bomb-making material past checkpoints in several instances.

Questionable rules. The TSA has issued 25 versions of screening procedures over the years, and there's still confusion about bringing liquids and gels aboard. It also allows items such as lighters, tools, corkscrews, and pointed scissors that could be used as weapons, just as box cutters were used in some of the Sept. 11 attacks.

Insecure cockpits. In easing the rules, the TSA pointed to other security measures, such as strengthening cockpit doors to deter hijackings. But Consumer Reports documented dozens of problems with those barriers, including doors popping open in flight, pilots being locked out, and flight attendants breaking the doors by slamming them shut.

Thin security forces. The government has tried to plug security holes in part by authorizing more flight crew members to carry guns. But the effort has lagged because of cumbersome training arrangements. And deployment of armed air marshals is hurt by staffing and morale problems, say several current and former marshals. Air marshals also complain that their reports of suspicious activity often seem to be ignored.

Although there has not been a successful terrorist attack in the U.S. since 9/11, dozens of security officials and others on the front lines say the security lapses make it easier for one to take place.

"The clock is ticking," says David Mackett, president of the Airline Pilots Security Alliance. "There's a term we are using in the airline industry: We are being ridden," he says of terrorists or their supporters whom he believes are continually riding planes and testing the system. "They're not wannabes. In some way, they're assisting Al Qaeda. People ask me, 'Will there be another 9/11?' I think there will be more 9/11s."

Clark Kent Ervin, inspector general for the U.S. Department of Homeland Security (DHS) from 2003 through 2004, says: "Al Qaeda is back in business. They're intent and they have this fixation on aviation."

Ervin's office criticized the TSA's oversight of security and its spending. He says it would be "very, very hard" for terrorists to commit an act similar to 9/11, but also says: "We Americans tend to fight the last war. I don't think we're much farther along, to tell you the truth."

Not everyone is so pessimistic. "We are significantly safer now than on Sept. 10, 2001," says Bob Hesselbein, chair of the National Security Committee for the Air Line Pilots Association, the nation's largest pilots' union. "Does that mean we're content? No. But we're in a much better place."

TSA spokesman Darrin Kayser says, "TSA has made significant strides in testing and deploying the latest technologies to better detect explosives and other potential threat items."

If outer defenses falter, federal auditors note, the authorities are counting on "able-bodied passengers" to engage in "self-defense actions should an incident occur onboard commercial aircraft."