Memorial weekend kicks off a summer of high-profile events in the U.S., from the dedication of the new World War II memorial in Washington to the two political conventions in Boston and New York.
Federal intelligence officials suspect Al Qaeda operatives already here are planning bombings using trucks and cars.
The head of the FBI told senators Thursday that there is cause for concern. In the meantime, the nation's passenger rail systems are going to have to comply with a new set of rules beginning Sunday.
The Department of Homeland Security is ordering a whole host of things like the removal or replacing of train station trash cans to using canine explosives detection teams.
Still, there are no current plans to move the country to orange-alert status. Security is already heightened for the summer travel season, with long lines expected at the nation's airports, and law enforcement is keeping an eye out for suspicious thefts like one last month in Pennsauken, New Jersey. An empty gasoline tanker truck was taken from an unsecured parking lot.
"We're concerned this could be used as a transport vessel for explosives or a bioweapons device," said Capt. Drew Lieb of the New Jersey State Police. "We're still actively pursuing recovery of it."
During the same time frame, there was another theft involving 3,300 gallons of a chemical used to make explosives.
Law enforcement officials say these types of reports are fairly routine, but they don't want to rule anything out.