The FBI is warning law enforcement officers around the United States to be on high alert for certain people connected to Al Qaeda (search) who may be planning to use a truck bomb.
The bulletin issued Wednesday night said local law enforcement should be alert to a person "of investigative concern" who possesses a commercial driver's license with authority to transport hazardous materials or who buys a "heavy vehicle" such as an ambulance, bus, van, or utility vehicle.
Counterterrorism officials told Fox News that they are especially concerned about the possibility of a truck bomb and said an influx of credible intelligence has been received suggesting that Al Qaeda may be contemplating such a bomb.
The bulletin repeated warnings issued Wednesday by the FBI and Justice Department for everyone in the United States to be on the lookout for certain individuals with links to Al Qaeda.
"For the next few months, we have reason to believe there will be a heightened threat to United States' interest around the world," Mueller said.
Between the 2 p.m. EDT press conference Wednesday and 9 a.m. Thursday, the FBI's Washington headquarters received 860 tips. That doesn't include any tips called into field offices.
"Our purpose is public awareness and certainly we got that," a law enforcement official told Fox News.
Wednesday's bulletin also offered some "Intelligence Priorities" to assist local law enforcement with the task of identifying potential terrorist activities.
"Al Qaeda continues to express a desire to conduct attacks in the United States," the FBI said. "Recent public statements by Al Qaeda leaders suggest that plans to attack the United States may be near complete."
So-called "targets of opportunity" may include the G-8 Summit (search) in Sea Island, Georgia; the national party nominating conventions in Boston and New York City; the November presidential election; and the Summer Olympics in Greece.
"The face of Al Qaeda may be changing. It is possible Al Qaeda will attempt to infiltrate young Middle Eastern extremists into America, as they did prior to September 11. Al Qaeda is a resilient and adaptable organization known for altering tactics in the face of security measures," the bulletin stated.
"The ideal Al Qaeda operative" may be in his late 20s or early 30s, the FBI said, and may travel with a family to lower his profile. Intelligence confirms that Al Qaeda is seeking out people who can portray themselves as Europeans, and the possibility that female operatives may be recruited cannot be ruled out.
Al Qaeda also attracts Muslim extremists among many nationalities and ethnicities, including North Africans and South Asians, as well as young Muslim converts of any nationality inside target countries, the FBI stated.
"Despite the capture of high-level operatives, al Qaeda continues to modify its techniques and operational plans to overcome U.S. security countermeasures," states the bulletin.
The FBI also said in the warning that it has developed the following "Intelligence Priorities" for state, local and tribal agencies.
— Identify any persons of investigative concern who may be in a place to facilitate an attack.
— Identify any person of investigative concern who has a commercial driver's license with authority to transport hazardous materials.
— Identify any purchases or modifications of heavy vehicles (like buses, armored cars and ambulances) by or for any persons of investigative concern.
— Identify any persons of investigative concern who are interested in the purchase of airplanes, rockets or electronic components that could be used to make an improvised explosive device.
— Identify any persons of investigative concern asking for help from or in contact with Islamic extremist groups.
— Identify any persons of investigative concern who may have traveled to or from or had contact with others in Afghanistan, Pakistan or Yemen within the past two years.
— Identify any suspected surveillance activities of potential targets for attack.
Fox News' Anna Stolley contributed to this report.