Republican and Democratic leaders in Congress call the attacks in London cowardly and barbaric, saying U.S. lawmakers stand with the people of Britain as they stood with Americans after the September 11, 2001 attacks in the United States.
Lawmakers also say the attacks should serve as a harsh reminder that the war on terrorism must continue, and of the weaknesses remaining to be addressed in mass transportation security.
Just after the 2004 terrorist bombings that struck commuter trains in Madrid, Spain, congressional committees held hearings on rail, bus and other transit systems across the country.
The conclusion many lawmakers reached was that the United States remains at high risk of attacks aimed at this vital, but vulnerable part of the U.S. infrastructure.
Massachusetts Congressman Ed Markey, the top Democrat on the Homeland Security Committee in the U.S. House of Representatives, says Thursday's attacks in London are another reminder that the U.S. government needs to pay more attention to transportation security.
Last year, members of Congress warned that the Madrid terrorist bombings were a "wake up call" for the U.S. government on the need for greater security in the nation's mass transportation system.
"After September 11, rightly or wrongly, we were able to say we never saw it coming. In this situation, we have seen it, we have seen what is coming, said Massachusetts Democratic Congressman Stephen Lynch. "And we can either choose to respond to it and develop a safe system of passenger and cargo rail in this country, or we can ignore it and suffer the consequences."
Many U.S. cities have instituted tighter security and emergency plans since the Madrid bombings. This includes the Washington Metro system, one of the nation's busiest.
However, because mass transportation is so open, instituting extremely strict passenger screening would be a daunting and expensive task.
Asa Hutchinson, undersecretary of the Department of Homeland Security, spoke last year about the challenges of keeping mass transportation running while trying to strengthen security.
"If we had particular terrorist intelligence, that they were targeting a particular subway station, or particular area, but it might be over two months, you don't want to close it [the system] down," he said. "You want to keep it running. Our objective is to keep the transit systems working for the passengers in a safe environment."
In contrast to billions of dollars directed to strengthening U.S. airline security since the September 11, 2001 al-Qaida attacks in the United States, much less has been spent by the federal government on mass transit security.
The London bombings are certain to give new momentum to legislative efforts to increase spending for rail and bus security.
One is sponsored by New York Democratic Senator Charles Schumer, who said Thursday the federal government needs to lead a coordinated effort, in addition to steps taken at the city and state level.
In the House, a number of lawmakers are sponsoring legislation to provide as much as $10 billion in federal money over the next five years.