WASHINGTON -- Federal agencies increased inspections at borders and of the food supply, put nuclear plants on heightened alert and stepped up the search for possible Iraqi terrorist "sleeper cells" in a move to war footing Thursday.

As the U.S. assault on Iraq intensified, Homeland Security Secretary Tom Ridge told members of Congress that there had been no incidents in the United States in retaliation.

He said the "code orange" alert status imposed this week would stay in effect.

FBI agents, in an attempt to head off possible retaliation for the war, began fanning out across the country to interview thousands of people born in Iraq, the agency said. The FBI also said it also wanted to try to prevent hate crimes against people of Iraqi heritage.

The FBI's 56 bureaus were put on around-the-clock operation.

"We are running down every lead, responding to every threat," Director Robert Mueller said.

Federal meat and poultry inspectors, in their routine inspections, began testing for chemical or biological agents that terrorists might use, said Jesse Majkowksi, head of the Agriculture Department's food security office.

Increased testing

The Food and Drug Administration increased testing for such potential agents in products it analyzes, agency officials said. The Centers for Disease Control was keeping a lookout in case doctors notice any unusual outbreak of diseases or symptoms associated with those agents.

On Capitol Hill, some congressional hearings were canceled or their agendas abbreviated as lawmakers focused on the war. Tours of the Capitol were suspended effective Friday but visitors can still get into the House or Senate galleries with a pass from a lawmaker's office.

At one hearing, senators were told that the government had stepped up its inspections of cargo vessels coming into the United States because of continued worries terrorists might use them to bring in a radioactive "dirty bomb" or other explosive.

But Asa Hutchinson, Homeland Security's undersecretary for border and transportation security, acknowledged that the country remains vulnerable because more than 6 million cargo containers arrive in America every year and not all of them can be checked.

U.S. officials want to improve cooperation with major foreign ports to more clearly pin down what is being shipped.

An additional 125 border guards were sent to the Canadian border to strengthen security and help traffic flow, although officials said they had no specific threat involving the northern border crossings.

Inspectors are doing more searches of vehicles and cargo and asking travelers more questions at the 300 border-crossing points, airports and seaports, said Robert Bonner, commissioner of the Bureau of Customs and Border Protection.

More than a half-dozen states have moved National Guard troops to nuclear power plants, according to an industry survey. Additional security measures were implemented at reactor sites when the national threat level was raised to code orange late Monday.

An intelligence report that terrorists might have targeted the Palo Verde nuclear power plant in Arizona led that state's governor to dispatch the National Guard to the 4,000-acre complex 50 miles west of downtown Phoenix.

Congressional and administration sources, speaking on condition of anonymity, said the initial concern about Palo Verde -- although significant enough to warrant the warning -- is now being largely discounted.

Energy Secretary Spencer Abraham told a Senate hearing, when asked about the Palo Verde alert, that "actions are being taken by all the appropriate agencies to address the concerns that have been raised."

In other security-related developments:

--Illinois senators asked that a no-fly zone be imposed over downtown Chicago to reassure citizens.

--Airports and airlines were told to post more law enforcement officers in and around terminals and airport perimeters. Some airports were conducting random vehicle inspections and restricting parking near terminals.

--The Coast Guard began escorting ships into port and stepped up patrols of waterways. It also established four special maritime security units of 100 guardsmen each, at undisclosed ports.

--Oil and gas pipeline operators were advised to take a series of actions to protect key, sensitive portions of the country's 2.3 million mile system of pipelines.

--City mass transit systems were urged to take additional precautions including careful positioning of trash containers and examination of baggage lockers.

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On the Net:

Transportation Security Administration: http://www.tsa.gov

Federal Aviation Administration: http://www.faa.gov

Transportation Department: http://www.dot.gov

Homeland Security Department: http://www.dhs.gov