The national terror alert level (search) was lowered from orange to yellow Friday afternoon, indicating that the nation has returned to an "elevated" state of alert for a domestic terror attack.

"The intelligence community has concluded the number of indicators and warnings that led to raising the threat level have decreased, and the heightened vulnerability associated with the Memorial Day holiday has passed," said Brian Roehrkasse, spokesman for the Department of Homeland Security.

The alert level was raised to "high" on May 20 following the deadly homicide bombings in Riyadh, Saudi Arabia (search), and Casablanca, Morocco (search).

At the time, the Bush administration called for increased security nationwide, saying it was concerned that the wave of attacks overseas could spread to the United States. Authorities described the intelligence pointing to a domestic attack as general in nature, with nothing credible suggesting a time, location, method or target.

"The U.S. intelligence community believes that Al Qaeda has entered an operational period worldwide, and this may include attacks in the United States," Homeland Security Secretary Tom Ridge (search) said on May 20.

Ridge said Friday that the nation must remain on alert despite the return to yellow.

"The lowering of the threat level is not a signal to government, law enforcement or citizens that the danger of a terrorist attack has passed," he said in a statement. "The U.S. intelligence community remains concerned that Al Qaeda is attempting to exploit our weaknesses and believes that the United States and its interests are still at a significant risk of terrorist attack."

Ridge said visible security serves as a strong deterrent for terrorism.

The threat level was lowered after consultations with members of the president's homeland security council. Members include Ridge, FBI Director Robert Mueller, Attorney General John Ashcroft and CIA Director George Tenet.

A homeland security official told Fox News that in the process of corroborating general threat information, some threats were "found to be not credible or not viable" given new security measures in place.

The nation usually is kept in a heightened state of alert for only a short period of time -- typically two weeks or so -- because DHS doesn't want to fuel complacency and keep raising the cost of added security.

Ridge said the United States is much more secure at Code Yellow now than it was a year ago.

"More and more of our partners in the federal, state and local governments and the private sector have adopted the homeland security advisory system and have identified protective measures to strengthen security and reduce the nation's vulnerability to terrorist attacks," Ridge said Friday.

"Know that your efforts are making a difference. Through this partnership, we send a signal to those who would do us harm that America stands alert, united, and ready."

Prior to May 20, the Bush administration had raised the terror alert level from yellow to orange three times, setting off a flurry of increased security measures by cities, states and businesses. Each time, the level was lowered back to yellow after a few weeks.

The alert was first raised just before the first anniversary of the Sept. 11 terror attacks. A high-level Al Qaeda prisoner then had suggested attacks were imminent on U.S. embassies in Southeast Asia. The alert went to orange, and several embassies were temporarily closed.

It was raised again on Feb. 7 of this year, around the time of the hajj (search), the Muslim religious pilgrimage. Counterterrorism officials had noted intelligence information pointing toward a possible attack around the time of the holiday.

It was then raised in March, when President Bush gave Saddam Hussein 48 hours to get out of Iraq before the U.S.-led coalition invaded the country.

The Department of Homeland Security said that evidence indicated that while Al Qaeda and those sympathetic to their cause were a principal threat, Iraqi state agents, Iraqi surrogate groups, other regional extremist organizations and ad hoc groups or disgruntled individuals might conduct terrorist attacks against the United States or U.S. interests abroad.

The U.S. government also feared that operatives working for the Mukhabarat (search), Saddam's intelligence service, would attempt bombings or other traditional terrorist-style attacks.

The orange alert was lifted after the serious fighting was declared over in Iraq.

The lowest two levels, green and blue, and the highest, red, have not been used since the color-coded alert system was put in place more than a year ago.

Fox News' Catherine Herridge and The Associated Press contributed to this report.