The Bush administration, intensifying pressure on Syria, endorsed Lebanon's call for a broader U.N. investigation into the assassinations of former Prime Minister Rafik Hariri and other critics of Syrian influence in the country.

The State Department also suggested Wednesday that the Security Council might consider tough sanctions if resolutions designed to reverse Syria's influence in Lebanon were disregarded.

"Then I would expect the Security Council would want to take a look at what further measures might be required and allowed," spokesman Sean McCormack said as a long and public U.S. campaign against Syria picked up speed.

A U.N. inquiry already has tentatively implicated senior Syrian officials in the assassination last February of Hariri, a dogged critic of Syria's influence in Lebanon.

Lebanese journalist and lawmaker Gibran Tueni, a critic of Syria who spent months in France fearing assassination, was killed in a car bombing Monday after returning home.

President Bush and Secretary of State Condoleezza Rice condemned the slaying.

The Lebanese government, meanwhile, has asked the Security Council to expand the U.N. investigation. "We certainly support the Lebanese government in making that request," McCormack said.

The United States and France have coordinated the U.N. pressure campaign on Syria, which has led to the withdrawal of Syrian troops from Lebanon. Bush administration officials are suspicious, though, that Syria maintains a grip on Lebanon by keeping intelligence operatives in the smaller, neighboring country.

Detlev Mehlis, the head the U.N. probe, has accused Syria of burning documents and intimidating witnesses relating to Hariri's assassination, McCormack said. "This is not the behavior of a government and a state that wants to assist the international community and the Lebanese people in finding out what happened, who's responsible," the spokesman said.

In order to maintain an international spotlight on Syria and its need to cooperate, McCormack said, "We think it is very, very important that the council act to extend the mandate of this investigation."