WASHINGTON - Despite a huge investment in security, the American aviation system remains vulnerable to attack by Al Qaeda and other jihadist terrorist groups, with noncommercial planes and helicopters offering terrorists particularly tempting targets, a confidential government report concludes.
Intelligence indicates that Al Qaeda may have discussed plans to hijack chartered planes, helicopters and other general aviation aircraft for attacks because they are less well-guarded than commercial airliners, according to a previously undisclosed 24-page special assessment on aviation security by the Federal Bureau of Investigation and the Department of Homeland Security two weeks ago.
But commercial airliners are also "likely to remain a target and a platform for terrorists," the report says, and members of Al Qaeda appear determined to study and test new American security measures to "uncover weaknesses."
The assessment comes as the Bush administration, with a new intelligence structure and many new counterterrorism leaders in place, is taking stock of terrorists' capabilities and of the country's ability to defend itself.
While Homeland Security and the F.B.I. routinely put out advisories on aviation issues, the special joint assessment is an effort to give a broader picture of the state of knowledge of all issues affecting aviation security, officials said.
The analysis appears to rely on intelligence gathered from sources overseas and elsewhere about Al Qaeda and other jihadist and Islamic-based terrorist groups.
A separate report issued last month by Homeland Security concluded that developing a clear framework for prioritizing possible targets - a task many Democrats say has lagged - is critical because "it is impossible to protect all of the infrastructure sectors equally across the entire United States."
The aviation sector has received the majority of domestic security investments since the Sept. 11 attacks, with more than $12 billion spent on upgrades like devices to detect explosives, armored cockpit doors, federalized air screeners and additional air marshals.
Indeed, some members of Congress and security experts now consider airplanes to be so well fortified that they say it is time to shift resources to other vulnerable sectors, like ports and power plants.
In the area of rail safety, for instance, Democrats are pushing a $1.1 billion plan to plug what they see as glaring vulnerabilities. "This is a disaster waiting to happen," Senator Joseph R. Biden Jr., Democrat of Delaware, said last week at a Senate hearing marking the one-year anniversary of the deadly train bombings in Madrid.
Still, the new aviation assessment, examining dozens of airline incidents both before and after the Sept. 11 attacks, makes clear that counterterrorism officials still consider the aviation industry to be perhaps the prime target for another major attack because of the spectacular nature of such strikes.
The assessment, which showed that the F.B.I. handled more than 500 criminal investigations involving aircraft in 2003, will likely serve as a guide for considering further security restrictions in general aviation and other areas considered particularly vulnerable, the officials said.
The report, dated Feb. 25, was distributed internally to federal and state counterterrorism and aviation officials, and a copy was obtained by The Times. It warns that security upgrades since the Sept. 11 attacks have "reduced, but not eliminated" the prospect of similar attacks.
"Spectacular terrorist attacks can generate an outpouring of support for the perpetrators from sympathizers and terrorism sponsors with similar agendas," the report said. "The public fear resulting from a terrorist hijacking or aircraft bombing also serves as a powerful motivator for groups seeking to further their causes."
The report detailed particular vulnerabilities in what it called "the largely unregulated" area of general aviation, which includes corporate jets, private planes and other unscheduled aircraft.
"As security measures improve at large commercial airports, terrorists may choose to rent or steal general aviation aircraft housed at small airports with little or no security," the report said.
The report also said that Al Qaeda "has apparently considered the use of helicopters as an alternative to recruiting operatives for fixed-wing aircraft operations." The maneuverability and "nonthreatening appearance" of helicopters, even when flying at low altitudes above urban areas, make them attractive targets for terrorists to conduct suicide attacks on landmarks or to spray toxins below, the report said.
The assessment does not identify who might be in a position to carry out such domestic attacks.
While law enforcement officials have spoken repeatedly about their concerns over so-called sleeper cells operating within the United States, a separate F.B.I. report first disclosed last week by ABC News indicated that evidence pointing to the existence of such cells was inconclusive.
The question of how well the government is protecting airline travelers surfaced again last month after the disclosure in a Sept. 11 commission investigation that in the months leading up to the attack, federal officials received 52 warnings about Al Qaeda and Osama bin Laden, some warning specifically about hijackings and suicide operations.
Federal officials now say they have taken a number of steps to tighten security for helicopters, chartered flights and the like in response to perceived threats, as they did last August in temporarily ordering federal security guards and tougher screening for helicopter tours in the New York City area.
Rear Adm. David M. Stone, an assistant secretary at the Department of Homeland Security who oversees the Transportation Security Administration, said that "the report validates T.S.A.'s sense of urgency in our daily efforts to secure aviation, and that same sense of urgency can be found in our work securing every other mode of transportation."
The report also sought to codify the various responsibilities for aviation security in the increasingly complex labyrinth of federal agencies, and it examined 33 terrorist plots against airplanes inside and out of the United States over the years.
Of the more than 500 criminal cases involving aircraft handled by the F.B.I. in 2003, two were hijackings in the United States involving flights from Cuba that landed in Florida. More than 300 episodes involved undeclared weapons or other problems at screening and security checkpoints, while 175 cases were triggered by on-board interference or threats against crew members, often involving alcohol.
In one case, a passenger sprayed perfume at a flight attendant "in a hostile manner," the report said.