And as CBS News Correspondent Bob Orr reports, at roll call in Chicago, the first district police tactical squad received specific orders.
"We're supposed to pay special attention to all water facilities, transportation facilities, utility hub stations like Con Ed," says Lt. Gary Szparkowski of the Chicago Police.
Five days after raising the nation's terror alert level from "elevated" to "high", federal officials warn al Qaeda cells inside the United States present a continuing threat.
"Within our own country the organization maintains the ability and the intent to inflict significant casualties in the United States with little warning," said FBI director Robert Mueller.
In New York and Washington, two cities already hit by al Qaeda, more police officers have been assigned to train stations and subways.
And all across America, airports are on higher alert for possible truck bombs.
High-rise building owners are being urged to secure heating and cooling systems so terrorists can't use them to spread biological or chemical agents.
Also for the first time, Homeland Security officials recommend citizens prepare "disaster supply kits" to include:
"I think the prospects of a terrorist attack that would require us to seal up a room with plastic and duct tape is low, very low, but not impossible." John Parachini, a terrorism analyst at Rand Corporation.
But, security experts say as long as al Qaeda remains capable of attacks, threats will be part of American life. The challenge for everyone is to find the right balance between preparation and paranoia.