Two Explosives-Rigged Cars Defused in London
One analyst said the bombers could be trying to send Brown a message.
"It's a way of testing Gordon Brown," said Bob Ayers, a security expert at the Chatham House think tank. "It's not too far-fetched to assume it was designed to expedite the decision on withdrawal (from Iraq)."
The U.S. government urged Americans abroad to be vigilant but officials said they saw no potential terrorist threat in the United States ahead of the July 4 Independence Day holiday. Homeland Security Secretary Michael Chertoff said there were no immediate plans to raise the U.S. national threat level, currently at yellow, or elevated.
New York strengthened its already tight security as a precaution, putting additional police in Times Square and the mass transit system.
"We're going to ramp up a little bit, but nothing dramatic," Mayor Michael Bloomberg said. "We'll take a little bit of extra precaution. Some of you will notice, some of you won't -- but we have to be cognizant."