WASHINGTON (CNN) -- The Transportation Security Administration will deploy special teams to help enhance transit system security in eight cities over the July Fourth holiday, officials said Tuesday.
Police stop cars Monday at a checkpoint set up at the entrance of Los Angeles International Airport.
The teams -- consisting of K-9 explosive detection units, air marshals, transportation inspectors and behavior detection officers -- will be deployed in New York; Washington; Boston, Massachusetts; Chicago, Illinois; Los Angeles, California; Philadelphia, Pennsylvania; Houston, Texas; and San Francisco, California.
Called Visual Intermodal Protection and Response teams, they will focus on ground transportation systems as well as airports in some of those cities.
TSA spokeswoman Lara Uselding in Chicago said the VIPR team there will work in mass-transit areas but not in the airport.
An official with the New York Police Department said there is no general or specific threat to that city, and the VIPR team's deployment is a precautionary move. "[We're] not aware of a potential threat," the official said.
TSA officials said there is no link between the deployment and the recent terrorist activity in the United Kingdom, and no specific intelligence regarding any imminent attacks on the eight cities. The cities were chosen because they are the largest and most densely populated metro areas, the officials said.
The deployment comes amid elevated security concerns and because of the expected high numbers of people using mass transit during the Independence Day celebrations.
The VIPR teams were created after the 2004 train bombings in Madrid, Spain, that killed 191 people, according to the TSA Web site.
"VIPR teams work with local security and law enforcement officials to supplement existing security resources, provide deterrent presence and detection capabilities, and introduce an element of unpredictability to disrupt potential terrorist planning activities," the Web site said.
Homeland Security Secretary Michael Chertoff said Monday that his department has taken extra security measures since the failed terror attacks in Britain on Friday and Saturday.
"We have ... taken some measures people will see, some they will not see, in increased personnel. Some of them obvious, some of them plainclothes. Pushing out the security perimeter, not only with respect to airports, but also mass transit and train stations as well," Chertoff said.
He said U.S. security officials are in close touch with their British counterparts.
"Obviously, any information that we get from them, we factor into our own analysis," he said. "If that means we have to make adjustments with respect to our security or be on the lookout for particular people, we would take those steps immediately."
Chertoff said this summer is a time of heightened vigilance for the TSA.
"We have seen attacks over the past several years during the summer. We have seen an increase in the number of public statements. So we are mindful of all of this as kind of general background, and it is causing us to be extra vigilant going into this summer," he said.
CIA Director Gen. Mike Hayden sent a memo to his staff reminding them about "staying on target" in the wake of the British incidents.
"Events in Great Britain since last Friday serve as a reminder -- if we ever needed one -- that this remains a dangerous world and that our work in defending America is as important as ever," he wrote in the memo, of which CNN obtained a copy from a U.S. intelligence source.
Hayden mentioned the debate over methods used in the fight against terrorists, but said the CIA had a clear mission to use all its lawful authorities to defend the United States.
"Some say elements of the current debate reflect the thinking of a pre-9/11 world. Don't worry about that. Keep your eye on our objective. For all of us at CIA, today's date is clear: It's always September 12th." E-mail to a friend
CNN's Jeanne Meserve contributed to this report.