A warning from the State Department of a worldwide increased threat of terrorism that may involve commercial aircraft, plus a warning from the Department of Homeland Security — nearly two years after 9/11, could it happen again?
The federal government has taken steps to improve security at U.S. airports and says it is constantly testing the effectiveness of those measures, but it won't tell anyone the results of the tests.
There is little doubt that security has improved, but some family members of the victims of the Sept. 11, 2001 attacks say people have a right to know how effective the measures have been.
"The airlines and the government are not giving us hard information as to how effective security is," said Stephen Push, whose wife died on the plane that hijackers crashed into the Pentagon. "They'll tell us we're making this improvement or we're making that improvement. It may sound nice, it may even look nice in the airport to see these people in uniform going through the bags, but they have not provided us with the hard information as to how effective it is."
Security experts agree that a lot has been done to plug the security holes that allowed 19 hijackers to get on four commercial airlines on the morning of Sept. 11, 2001, but more could be done, they say.
"Is it harder than before 9/11? Yes," said Brian Jenkins of the Rand Corporation, a Washington think tank on security and international affairs. "In my view, is it hard enough? No, we have a ways to go."
Among the steps that have been taken at airports across the country:
The federal government has taken over security checkpoints, screeners are better trained, and in the last 18 months those screeners have confiscated 6 million banned items.
Checked luggage is now examined for explosives by machine, by hand or by trained bomb-sniffing dogs.
Every large passenger plane in the United States now has a cockpit door designed to withstand bullets and small explosives.
Thousands of air marshals have been trained and deployed.
Pilots are being trained to carry guns in the cockpit.
Workers who have access to the tarmac now must undergo background checks.
"Your flight is more secure because of layers of security, passenger screeners, baggage screeners, other things we're looking at," Department of Homeland Security spokesman Gordon Johndroe said. "No-fly lists to make sure people don't fly who shouldn't. It's not just what we're doing right on the airplane you're getting on. It's everything we're doing to keep people from getting near this country or this aircraft."
Despite the government effort, critics say security gaps remain, including:
Cargo security, because about one-fifth of all the cargo in the United States is shipped on passenger planes, but less than 2 percent of it is screened for explosives.
Even with all the new air marshals, they are present on very few of the nation's 35,000 flights a day.
Flight attendants, the first line of defense against terrorists, have still had very little if any training to take on would-be hijackers.
The government has yet to complete a sophisticated passenger profiling system designed to flag possible terrorists.
Meanwhile, Jenkins says terrorists have been working to try to keep up with the advances in security.
"They have their equivalent of an R and D program to look for ways to smuggle bombs past our detection systems," he said.
Ultimately, it may be passengers themselves who offer the best defense. These days, there is little doubt passengers would rise up against any hijacker.
Now, terrorism experts warn that as time goes on without an attack, the greatest vulnerability in the security system may be complacency — that government officials, airlines and passengers will let their guard down. They say that is something Americans can never afford to do.
ABCNews' Lisa Stark contributed to this report.