While it's unclear who was responsible for the apparent car bombs discovered Friday in London, this latest incident is adding to mounting concern by European and U.S. intelligence over potential attacks on Western targets.
"At this point, I have seen no specific, credible information suggesting that this incident is connected to a threat to the homeland," said U.S. Homeland Security Secretary Michael Chertoff. "We have no plans at this time to change the U.S. threat level."
But the Department of Homeland Security and the FBI are cautioning the public to remain vigilant, and released a bulletin to the law enforcement community Friday evening containing more information on the thwarted bombings.
The memo also describes suspicious behavior officers can watch for on the ground that could signal the possibility of a similar attack. Questionable activities could include theft or cash purchases of large cars, cargo vans and trucks, cash purchase of large containers or barrels, or sales of high quantities of chemicals or fuel.
Additionally, authorities advise that erratic driving seen near crowds or densely populated areas or abandonment of vehicles near such locations -- similar actions to what witnesses report seeing in London -- could be red flags.
As for this most recent London plot, the FBI is working with British authorities to pin down the vehicle registration and uncover the identities of any individuals who might be connected to the foiled plot, so they can see if there is a U.S. nexus.
Forensic evidence obtained by British authorities from the two Mercedes vehicles rigged with potential explosives will be run through a massive database U.S. authorities have been developing that tracks foreign nationals entering the United States.
"We have been in close contact with our counterparts in the U.K.," said Chertoff. "Our law enforcement and intelligence officials are closely monitoring the ongoing investigation."
ABC News has learned that a growing number of law enforcement and intelligence officials are increasingly concerned that al Qaeda or like-minded extremists are stepping up plans to attack targets in Europe, North Africa, Afghanistan and Pakistan. Some of these individuals have been at training camps in remote locations in the tribal areas of Pakistan, according to intelligence officials.
It is in those regions that authorities believe senior al Qaeda leaders are hiding. "They are plotting and planning," said one senior intelligence official.
In the past several months U.S. officials have been more concerned about attacks against U.S. interests around the world. A public announcement "Worldwide Caution" from the State Department updated in April noted, "Current information suggests that al-Qaida and affiliated organizations continue to plan terrorist attacks against U.S. interests in multiple regions, including Europe, Asia, Africa and the Middle East."
Sources have told ABC News that recent intelligence indicates a significant number of Islamic radicals are flowing into terrorist camps in Pakistan for training, then leaving for unknown countries.