Since late Wednesday, counterterrorism officials have chased a tip that al-Qaida may have sent three men to the U.S. on a mission to detonate a car bomb in either city. At least two of those men could be U.S. citizens, according to the tip.
No intelligence supported that tip as of Saturday, and officials continued to question the validity of the initial information.
While such tips are common among intelligence agencies, this one received more attention, and government officials chose to speak publicly about it, because of the connection to the anniversary of the worst terrorist attack in U.S. history.
Al-Qaida long has hoped to strike again on the anniversary. At the FBI field office in Washington, assistant director James McJunkin described the tip and the response as routine. The U.S. already had bolstered security nationwide before the upcoming anniversary and anticipated an increase in tips.
"We expect we're going to get an increase in threats and investigative activity around high-profile dates and events," he said. "This is a routine response for us. It's routine because it's muscle memory."
Intelligence analysts have looked at travel patterns and behaviors of people who recently entered the country. While they have singled out a few people for additional scrutiny, none has shown any involvement in a plot, according to the senior U.S. officials, who insisted on anonymity to discuss the investigation.
President Barack Obama met with his national security team Saturday, but the White House released no new information about possible threats. A statement said that counterterrorism efforts were working well and would not ease in the weeks and months ahead.
The tip that touched off the most recent investigation came from a CIA informant who has proved reliable in the past, according to U.S. officials. They said the informant approached intelligence officials overseas to say that the men were ordered by new al-Qaida leader Ayman al-Zawahri to mark the 10th anniversary of the Sept. 11 by doing harm on U.S. soil.
He took over as the group's leader after the U.S. killed Osama bin Laden during a raid in May at his compound in Pakistan.
The informant said the would-be attackers were of Arab descent and might speak Arabic as well as English. Counterterrorism officials were looking for certain names associated with the threat, but it was unclear whether the names were real or fake.
Some intelligence officials have raised doubts about the threat, given the short turnaround time. Someone who recently arrived in the United States would have just days to plan and obtain materials for a car bomb attack, a difficult feat even with a long lead time.
But they did not dismiss the threat. Extra security was put in place to protect the people in the two cities that took the brunt of the jetliner attacks that killed nearly 3,000 people at the World Trade Center and the Pentagon.
Law enforcement agencies around the country had increased security at airports, nuclear plants, train stations and elsewhere in the weeks leading to Sept. 11. The latest threat made those measures more urgent.
U.S. embassies and consulates also stepped up safeguards in preparation for the anniversary.
While authorities urged people to keep a watchful eye for suspicious activity as usual, they said there was no reason the latest tip should change anyone's weekend.
"Whatever you have plans for, it's a beautiful day. It's going to be a beautiful weekend," McJunkin said. "It's college football Saturday. Tomorrow is the start of the NFL season. So we expect the public is going to be out enjoying what it means to be an American."
Associated Press writers Eileen Sullivan, Kimberly Dozier, Jessica Gresko and Lolita C. Baldor contributed to this report.