But several U.S. embassies and consulates remained closed Thursday, on watch for possible terrorist attacks.
The U.S. military increased fighter jet patrols over 10 cities, and missile launchers around Washington were armed Wednesday.
The White House homeland security director, Tom Ridge, spent the day monitoring developments around the country after he returned from a memorial service in Shanksville, Pa.
"So far, so good," he said.
Reports of strange passenger behavior led two commercial flights to make unscheduled landings Wednesday. Three men on one of the jets were arrested, but authorities said neither incident appeared to be related to terrorism.
The Air Force Academy was closed to the public because of an unspecified security concern, and there were no immediate plans to reopen its gates. Officials ordered the shutdown of the 19,000-acre campus Tuesday night.
There was no specific threat but officials decided they needed to increase security, academy spokesman Lt. Greg Hignite said. "At this point, it's indefinite," Hignite said of the closure. The academy is one of Colorado's most popular tourist attractions.
The military command in charge of operations in the Middle East and Central Asia moved to the Pentagon's highest security level - known as "Delta" - after receiving credible threats from more than one country. Officials declined to elaborate.
The order followed one by the Bush administration on Tuesday that the color-coded domestic alert be raised to its second-highest level - orange - based on new intelligence warning of possible strikes, mostly overseas.
Information prompting the alert across America came from Omar al-Farouq, a senior al Qaeda figure captured in Asia, according to two government sources who spoke Wednesday on the condition of anonymity.
Al-Farouq, an Arab who was described as an al Qaeda operations chief in Southeast Asia, has been in U.S. custody since this summer and provided his interrogators specific information suggesting that terror cells in the region were planning attacks on U.S. facilities, the sources said.
Other unspecified intelligence appeared to corroborate al-Farouq's claims. He had been based in Indonesia. Al-Farouq's name was first reported by The New York Times.
In response to the alert, the military resumed round-the-clock combat patrols over Washington and New York on Friday, and expanded to include 10 more cities, defense officials said. Fighter jets also were on alert on airstrips at more than a dozen other locations, ready to scramble if needed.
U.S. embassies or consulates in Germany, Denmark, Italy and Luxemburg received letters containing white powder on Wednesday, sparking fears of a fresh anthrax attack, a U.S. State department official said.
But the unidentified substances in letters sent to U.S. embassies in Hamburg, Copenhagen and Luxembourg on Wednesday were found to be harmless in preliminary tests, officials said. The one in Hamburg was found to be sugar, officials said.
And a suspicious powder found in a letter sent to the U.S. consulate in Leipzig on the anniversary of the September 11 attacks was found not to be anthrax, a spokesman at the U.S. embassy in Berlin said on Thursday.
Troops with semiautomatic rifles were posted outside the U.S. Embassy in Indonesia, the world's most populous Muslim nation. The building remained closed Thursday, along with U.S. missions in Cambodia and Malaysia. Officials were unable to say when they would reopen.
In Indonesia, U.S. ambassador Ralph Boyce met Vice President Hamzah Haz to discuss the security threats, which have raised tensions between Washington and Indonesian officials concerned their country is getting a reputation for terror.
Boyce denied claims by Indonesian authorities that the embassy had not informed them of the alert.
"There was not much time," he said. "It was a fast breaking event. We notified our counterparts in Jakarta before we even notified Washington."
In the Philippines, the British Embassy was evacuated after a staffer there received a bomb threat. No explosives were found, but the building was closed until Friday.
Southeast Asia has been dubbed the second front in the U.S. led war on terror. Several dozen suspected al Qaeda operatives have been arrested in the region over the last year, and other senior figures in the network reportedly have been deported to the United States.
In other developments:
source said they were suspected of belonging to al Qaeda.