Federal investigators are pursuing leads in the United States related to the foiled plot to blow up flights from Britain but so far have found no evidence of terrorist activity, Bush administration officials said Friday.

Homeland Security Secretary Michael Chertoff, meanwhile, said he would adjust new flight restrictions to try to make air travel for passengers "as simple and as easy as possible, as quickly as possible."

U.S. counterterrorism officials stressed that there was no reason for Americans to believe plotters or others connected with the terror scenario were in the country. Still, the FBI has so far assigned an estimated 200 investigators to track down potential links.

"We operate on the assumption that we don't have everybody," White House homeland security adviser Frances Fragos Townsend said in an interview with The Associated Press. "I never, and I don't think anybody else in this business ever assumes when you take a case down that you've gotten everybody."

"I think it's pretty clear that in this case, we don't have everybody," she added.

In the two weeks before authorities cracked the case, leading to 41 arrests in Pakistan and Britain, the FBI asked for legal permission in a significant number of cases to obtain records, conduct searches or surveillance in the United States. So far, the increased scrutiny has not turned up terror-related evidence, according to a federal law enforcement source.

The arrests led the Bush administration to put the U.S. on its highest threat alert for flights headed to the United States from Britain. Additionally, all other flights were raised to the second-highest alert level.

Two other U.S. counterterrorism officials, speaking on condition of anonymity because of the sensitivity of the investigation, said the British suspects placed calls to several cities in the United States before their arrests. At least some of the calls were placed to people in New York, Washington, Chicago and Detroit, one official said. The suspects are all British citizens, mostly men in the 20s and 30s of Pakistani descent.

The FBI is expecting the arrests and searches of homes and computers in England to generate another round of leads on possible U.S. ties. But there have been no arrests in the United States in connection with the plot, officials said.

Dozens of Muslim and Arab people in the United States reported being questioned by law enforcement officials over the last several weeks, community leaders said. But they believed the scrutiny was related to the conflict between Israel and Lebanon — and not to the British-based plot.

"We were all looking one way," said James Zogby, president of the American Arab Institute, who said he personally was told of only a handful of recent complaints. "But if something else was happening, we weren't focused on it."

Authorities on both sides of the Atlantic fear there still could be dozens of people loose who participated in the plot — even if only tangentially, like by supplying chemicals or funding. Lawmakers also continued to be briefed on details.

Rep. Pete Hoekstra, R-Mich., chairman of the House Intelligence Committee, interrupted an annual bike tour in his district to return to Washington and meet with U.S. officials. Rep. Rob Simmons, R-Conn., who also was briefed Friday, said he believed the Patriot Act and other laws that permit domestic surveillance of terror suspects were critical to capturing the suspects.

Rep. Peter King, chairman of the House Homeland Security Committee, said he was assured during several intelligence briefings that "there's absolutely no evidence" of plotters in the country.

"They're not looking at anybody in the U.S.," said King, R-N.Y.

Chertoff, seeking to ease travel concerns at Washington's Reagan National Airport, also sought to assure Americans that there is no evidence of plotters in the United States. But he cited the possibility of other terrorists or sympathizers, saying, "So I'm not prepared to let my guard down."

The Transportation Security Administration has barred many common carry-on items such as water bottles and toiletries since the plot was unraveled. Chertoff provided no details but said officials would focus on the threat involved — some common chemicals, which combined can form a deadly explosives.

Officials will study "how we can calibrate our systems to take account of these developments, and then, with that in mind, try to ultimately come back to a regime of security that will give the maximum amount of freedom to the travelers," he said.