Enlarge By Matt Rourke, AP A jet lands near a United Parcel Service jet that is isolated on a runway at Philadelphia International Airport on Friday. Law enforcement officials are investigating reports of suspicious packages on cargo planes in Philadelphia and Newark, N.J. Obama: Suspicious packages are 'credible terrorist threat' WASHINGTON  Two suspicious packages sent from undisclosed sources in Yemen to synagogues in Chicago tested positive for explosives after they were intercepted by counterterrorism officials in the United Kingdom and Dubai, President Obama said Friday. Obama described the incident as a "credible terrorist threat against our country" and said U.S. officials had launched a wide-ranging investigation to determine the scope of the threat. John Brennan, the White House adviser on homeland security and counterterrorism, declined to elaborate on the quantity or type of explosive materials detected in the devices. VIDEO: Obama on discovery of 'explosive material' RESPONSE: Yemen government suprised at link to explosive packages "We don't want to presume that we know the bounds of this plot," said Brennan, adding that heightened precautions were being taken to screen cargo and other material entering the country from Yemen, a hotbed of terror activity and an active base for terror operatives associated with al-Qaeda. He said authorities don't know yet whether the packages represented an actual attempt to attack or a possible dry run to test security for a future attack. Brennan said he has spoken with Yemeni government officials and that they had pledged their cooperation in the U.S. investigation. The discovery triggered a frenzy of cargo searches Friday in New York, New Jersey and Pennsylvania. U.S. fighter jets, meanwhile, escorted a commercial aircraft to landing at New York's John F. Kennedy International Airport. There was no known threat associated with the United Arab Emirates flight, the FBI said in a written statement. But the FBI said the aircraft was carrying cargo from Yemen. Navy Lt. Cdr. Bill Lewis said the jets were guiding the plane "out of an abundance of caution."' The broad search in the U.S. was launched after a package from Yemen raised red flags in the United Kingdom late Thursday night, local time. White House spokesman Robert Gibbs said Friday that President Obama was notified of a "potential terrorist threat" at 10:35 p.m. ET Thursday by Brennan. Gibbs said intelligence and law enforcement agencies discovered potential suspicious packages on two planes in transit to the United States. "Based on close cooperation among U.S. government agencies and with our foreign allies and partners, authorities were able to identify and examine two suspicious packages, one in London and one in Dubai," Gibbs said. "Both of these packages originated from Yemen. As a result of security precautions triggered by this threat, the additional measures were taken regarding the flights at Newark Liberty and Philadelphia international airports." In New York, police Commissioner Ray Kelly said Friday that investigators located one package in the back of a parcel truck in New York. He said the package was actually an envelope containing bank receipts from Yemen intended for JPMorgan Chase. "The package has been cleared," Kelly said, adding that another similar-looking package in the truck was checked and cleared of any potential threat. The incidents prompted the Department of Homeland Security to announce that it was bolstering security. "As a precaution, DHS has taken a number of steps to enhance security," the agency said in a written statement. "Some of these security measures will be visible while others will not. The public may recognize specific enhancements including heightened cargo screening and additional security at airports." The statement indicated that passengers should "continue to expect an unpredictable mix of security layers that include explosives trace detection, advanced imaging technology, canine teams and pat-downs, among others." "As always, we remind the public to remain vigilant and report suspicious activity to local law enforcement." UPS confirmed Friday afternoon that cargo on three of its planes had been investigated by authorities. "UPS is fully cooperating with authorities who are investigating reports of potentially suspicious packages onboard cargo flights," said UPS spokeswoman Kristen Petrella. "We can confirm that authorities have investigated two (UPS) aircraft in Philadelphia and one in Newark." "All of those aircraft have landed safely," she said. "Security is of the utmost importance to UPS." Yemen, the source of the suspicious packages, has become a hotbed of terrorist activities. Al-Qaeda, chased from training grounds in Afghanistan and Pakistan, has established a base there where terror operatives, including U.S. citizen Anwar al-Awlaki, have allegedly sought to assist or plot attacks against the U.S. Umar Farouk Abdulmutallab, charged in the Christmas Day attempted bombing of a commercial airliner over Detroit, is alleged to have received training in Yemen. Maj. Nidal Hasan, the accused gunman in last year's massacre at Fort Hood, Texas, is alleged to have sought guidance from al-Awlaki. Contributing: Tom Vanden Brook in Washington Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more