Local officials around the United States have expressed condolences to the victims of the bombing attacks in London. They have also increased security at train stations and airports.
Los Angeles Mayor Antonio Villaraigosa said the "hearts and prayers" of his city are with the London victims.
"On behalf of the people of the city of Los Angeles, we extend our heartfelt sympathies and sorrow for those who were injured or killed, and to their families and loved ones," said Antonio Villaraigosa.
New York Governor George Pataki spoke in New York City, which was the target of terror attacks September 11, 2001. Mr. Pataki extended his sympathies to the people of London and said Thursday's attacks remind everyone that the war on terrorism is not over.
"We know that an attack on innocent civilians, the vicious, cowardly attack that occurred in London today, is an attack on every one of us, every one of us who believes in freedom and who believes in our ability to choose our own destiny, and not be told by others how to live our lives," said Governor Pataki.
U.S. officials say they have no information that the U.S. transit system is threatened, but they have heightened security on buses and trains around the country as a precaution. New York Mayor Michael Bloomberg says authorities have also increased surveillance of bridges, tunnels, and at the city's port. Los Angeles has added additional officers at regional airports.
San Francisco officials have added officers to the Bay Area Rapid Transit system, which serves 300,000 commuters a day. And there are additional police on the streets in Washington DC.
Los Angeles Sheriff Lee Baca met reporters early Thursday, along with the city's police chief, William Bratton. Mr. Baca echoed the comments of President Bush, who said the United States will not yield to terrorism, and of British Prime Minister Blair, who promised that terrorism will be defeated.
Mr. Baca asked residents to be on the lookout for suspicious activity.
"This is a very unfortunate reality that we're into now," said Sheriff Baca. "And Los Angeles stands firm with the president of the United States. We stand firm with the Prime Minister of Great Britain. We are in contact with our colleagues worldwide."
He says local police departments in cities around the world are sharing intelligence.
New York Governor Pataki urged residents to go about their business as usual.
"So take the subway, ride the train, go to work, play in the parks," he said. "The best security in the world is there to protect you."
The mayor of Los Angeles says he will ride the city's buses to reassure passengers that they are safe.