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 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.

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.

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.