WASHINGTON -- The Bush administration lowered the terror alert level from orange to yellow Wednesday, saying the end of heavy fighting in Iraq has diminished the threat of terrorism in the United States.
It was unclear whether the heightened alert had prevented any terrorist attacks, officials said.
A number of security measures around the country will be relaxed, officials at the Department of Homeland Security said.
For example, the federal "Operation Liberty Shield" -- the code name for security measures keyed specifically to the war on Iraq -- will be terminated. It called for National Guard or state police to be posted at nuclear power plants and other critical facilities.
Still, a significant threat remains, officials said.
The new, yellow level marks an elevated risk of terrorist attacks. It is the middle level on the five-tier danger scale. The old level, orange, signifies a high risk and is the second-highest level. A red alert has not been declared since the system was begun 13 months ago.
"We must be vigilant and alert to the possibility that al-Qaida and those sympathetic to their cause, as well as former Iraqi-regime state agents and affiliated organizations, may attempt to conduct attacks against the U.S. or our interests abroad," Homeland Security Secretary Tom Ridge said.
The threat level was raised on March 17, two days before the war began.
"We believe that during Operation Liberty Shield there were individuals in places, at times, where they should not have been," said Brian Roehrkasse, a spokesman for the Department of Homeland Security. "The investigations continue on those."
Roehrkasse declined to provide specifics.
President Bush agreed to lower the level during an intelligence briefing Wednesday on Ridge's recommendation, said White House spokesman Scott McClellan.
The Transportation Security Administration told airports they could discontinue the random vehicle inspections and parking restrictions put in place at code orange.
Not everyone was relaxing.
The New York Police Department said the city would remain on orange alert and checkpoints at bridges and tunnels would continue.
"We are maintaining the current status because New York remains under a greater risk of terrorism than other parts of the country," police spokesman Michael O'Looney said.
The alert system is designed to guide law enforcement agencies, businesses and the general public in their security decisions, and it is mostly up to local governments and companies to decide what measures to enact.
Those outlets must also pay for special security, although the federal government is directing billions to support the efforts.
The color level is driven by world events and information gathered by U.S. intelligence agencies, such as monitored terrorist "chatter" that sometimes increases before an attack.
The alert level has bounced between yellow and orange since the system was put in place in March 2002. Like the highest-level red, the lowest levels of blue and green have never been used.
No domestic attacks have taken place since Sept. 11, 2001.
For this alert, U.S. counterterrorism officials say the most specific information pointed to possible attacks on U.S. forces in the Middle East. A recent statement from Osama bin Laden, the Saudi-born leader of al-Qaida, declared some solidarity with Iraqis, although he also criticized Saddam Hussein's secular government.
In addition, it was feared that people working for Iraq's intelligence service would attempt bombings or other terror attacks, officials said. Such people are thought to work out of Iraqi embassies around the world under diplomatic cover.
Shortly before the war, the administration asked 60 countries to expel some 300 people it alleged were Iraqi intelligence operatives. Many countries, their governments opposed to the war, refused.
Alleged Iraqi operatives were arrested in Jordan and Yemen in connection with plots against American embassies there. And the Philippines expelled several Iraqis for alleged links to espionage and Islamic extremist groups.