They do stress that they have not detected any credible threats so far, but that could change at any moment.
Security is heightened locations such as 30th Street Station in Philadelphia. There, TSA agents and Amtrak police have been highly visible.
Authorities are concerned about home-grown freelancers, as opposed to a massive attack like the one of 9/11. Security dogs were on patrol and police were sampling carry-on bags with mobile device to detect explosives.
While some passengers at 30th Street Station may have been unnerved, most were reassured by the extra security.
"We've been watching them walk around and they've got dogs sniffing everything, so we actually feel real safe," said Kathy West of Detroit, Michigan.
When asked if she worried about travel, Ilona Prishack of Pittsburgh said, "A little. But I worry all the time, though."
Philadelphia police continue to monitor potential targets with mobile Homeland Security units, focusing on Center City and the Independence Mall area, with its concentration of historic attractions and potential economic targets including the U.S. Mint and the Federal Reserve Bank.
The Department of Homeland Security is now using a specific alert system instead of color codes. So far, they haven't issued any alerts. However, Great Britain has raised its alert level to "severe," meaning an attack is highly likely.
Philadelphia police have previously said they had stepped up patrols at transportation hubs, religious sites, historic places and other public areas.
