WASHINGTON (CNN) -- The Department of Homeland Security is warning that Islamic extremists might be plotting suicide airliner hijackings to be carried out before the end of the summer, with possible targets including sites in Britain, Italy, Australia or the eastern United States.
"As of mid-June, Islamic extremists may have been planning suicide hijackings to be executed by the end of Summer 2003," according to the document obtained Tuesday by CNN.
"The plan may involve the use of five-man teams, each of which would attempt to seize control of a commercial aircraft either shortly after takeoff or shortly before landing at a chosen airport."
Such a plan would preclude the need for hijackers to take flight training, the document says.
The one-page warning, sent to U.S. airlines and others Saturday, contains words of warning and skepticism, saying some of the intelligence the government has received raises "questions about the viability of the plot."
Homeland Security sources said the credibility of the information is still being evaluated. One government official termed it "dubious."
Intelligence leading to the advisory was developed during the past several weeks and is based, at least in part, on information obtained by interrogating high-level al Qaeda detainees, according to a senior intelligence official.
"Attack venues may include the United Kingdom, Italy, Australia, or the East Coast of the United States due to the relatively high concentration of government, military and economic targets," the advisory says.
Britain, Italy and Australia were allies of the United States in the war against Iraq.
According to the advisory, possible targets could include national symbols, economic targets and venues that would produce many casualties.
Also Tuesday, the State Department reissued its worldwide caution reminding U.S. citizens of the threat of terrorist acts, citing tensions remaining from the war in Iraq as a possible reason terrorists and other groups might strike.
"Terrorist actions may include, but are not limited to, suicide operations, hijackings, bombings, or kidnappings," the caution said, adding, "These may also involve commercial aircraft."
Other potential modes of attack include conventional weapons like explosives, and nonconventional weapons like chemical and biological agents, the department said.
The agency also reminded Americans that terrorists "do not distinguish between official and civilian targets," which might include places like restaurants, houses of worship, schools, hotels and beaches at which U.S. citizens and foreigners congregate.
Such worldwide cautions are issued periodically.
Warning of new terror strategies
The Department of Homeland Security warning also says the hijackers might try to use everyday travel items, such as cameras, that have been modified into weapons.
"The hijackers may try to calm passengers and make them believe they are on a hostage, not suicide, mission," it reads.
By doing this, one official said, the hijackers might be hoping to reduce the likelihood that passengers would take pre-emptive action, as those aboard United Airlines Flight 93 did September 11, 2001, after which the plane went down in a field in Pennsylvania.
But that possibility, the advisory says, "seems unrealistic given the widely publicized actions of passengers on the United Airlines Flight 93."
The advisory also says the terrorists might plan to identify flights that transit through the United States or the other target countries, so that the hijackers would not need visas for those countries.
Although the warning lays out specifics of the possible plot, it also says, "No equipment or operatives are known to have been deployed to conduct the operations."
Homeland Security officials said there is no immediate plan to raise the nation's threat level, but that the aviation sector has been encouraged to review security practices and implement new random measures.
One official would not specify what the new security steps might be, but said one of the points of the advisory is to encourage security personnel at every level to be on the lookout for new and different terrorist techniques.
FBI Director Robert Mueller spoke Tuesday in Arlington, Virginia, to about 1,000 police officers, firefighters and other first responders. In his speech, he did not mention the new warnings, according to The Associated Press.
Mueller said the United States, while still facing the threat of terrorism, is safer now than it was on September 11, the AP reported.
"The challenge for us is to recognize the threat is continuous out there and to do everything we can to prevent the next attack," the AP quoted Mueller as saying. "The oceans that separate the United States are not sufficient to prevent other attacks."
Investigation report
Last week, a congressional report stated that U.S. intelligence agencies, including the FBI and CIA, "missed opportunities" to disrupt terrorism before September 11. (Full story)
According to the report, senior military officials were poised to attack terrorist mastermind Osama bin Laden before September 11 but lacked the intelligence data they needed to execute their plans.
The report also questions Saudi Arabia's cooperation in earlier terror probes and examines its link to the hijackers. (Saudi official, Bush meet on report)
The roughly 800-page report does not point to any single clue that could have thwarted the plot, but says "various threads and pieces of information" were either overlooked or not put together.
"No one will ever know what might have happened had more connections been drawn between these disparate pieces of information. ... The important point is that the intelligence community, for a variety of reasons, did not bring together and fully appreciate a range of information that could have greatly enhanced its chances of uncovering and preventing bin Laden's plan to attack the United States on September 11, 2001," the report says.
After the attacks, Congress approved the creation of a Department of Homeland Security to increase cooperation between the CIA and FBI and serve as an umbrella for other agencies that deal with issues of domestic security.
In a written statement last week, Mueller thanked the joint committee for the report, saying it would generate a "constructive discussion." He also said the agency has already implemented or is in the process of implementing many of the recommendations on improving counterterrorism
