This is the first time in two decades that U.S. interests in Lebanon have been directly targeted, and this attack comes as the latest in a wave of bombings against politicians and journalists followed the assassination of former Prime Minister Rafik Hariri in February 2005.

In the 1980's, at the height of the 1975-1990 civil war, in separate events the U.S. embassy and marine barracks were blown up, hostages were taken, and a TWA airliner was hijacked.

Security sources said Tuesday's blast was triggered by a stolen booby-trapped car, which was laden with 20 kilograms (44 pounds) of TNT and parked on the side of a road. Two Lebanese and one Syrian passerby were killed and 26 others injured, including the driver of the U.S. embassy car, and an American national who was nearby by chance.

The embassy in Beirut said U.S. inspectors and Lebanese authorities are investigating the attack. Embassy staff have since had their movements restricted and U.S. citizens in the country were urged to exercise "extreme caution."

Lebanese commentators and politicians on Wednesday widely speculated on the "messages" the attack carried, in terms of its target and timing.

Some suggested it was a message for U.S. President George W. Bush, who wrapped up his eight-day Middle East tour in Egypt Wednesday, on a trip that was widely seen as a mission to support Israel and subjugate any opposition to Tel Aviv and Washington in the region.

Lebanese analyst Ahmed Mosali, a political science professor at the American University of Beirut, told the Middle East Times in a telephone interview the attack was a "clear message to the American administration, saying that its policies in the region are wrong."

The Lebanese Hezbollah-led opposition accuses the United States, through its diplomats, of fueling the internal crisis between the ruling pro-Western March 14 Alliance and the opposition, some of whom are backed by Syria and Iran. The two camps have been locked in dispute over electing a president. The country has been without a head of state since pro-Syrian Emile Lahoud's term expired on Nov. 23.

"The implications for the U.S. in the political power game in Lebanon are huge," remarked Mosali. "And the conflict between Washington and Tehran is unfolding in Lebanon."

Speaking at Egypt's Sharm el-Sheikh resort on Wednesday, Bush told Syria, Iran and their allies to "end their interference and efforts to undermine" the election of a president.

Hezbollah chief Hassan Nasrallah denounced the latest attack during a religious ceremony on Tuesday night, warning that such bombings "can lead to negative consequences and will reflect on the country's stability."

Politicians and analysts on all sides agreed that Tuesday's blast was a serious setback for the fragile stability of the country, where already tight security was beefed up still further with new army checkpoints.

"Someone does not want a president or diplomats in Lebanon," Lebanese Information Minister Ghazi Aridi, from the ruling March 14 Alliance, told the Middle East Times. "They want to take us back to the years of the 1980s when chaos and instability prevailed."

Aridi's anti-Syrian group has previously accused Damascus for the series of bombings. But no one has pointed the finger at any one party this time, while the number of regional and international players wading into this small, crisis-ridden Arab country is growing.

Many Lebanese commentators believed the blast, which took place less than 24 hours ahead of a visit by Arab League's chief Amr Moussa, was an attempt to torpedo the organization's efforts to resolve the presidential crisis.

Moussa arrived in Beirut Wednesday to try to persuade all parties to accept a plan, endorsed by the 22 Arab foreign ministers members in Cairo on Jan. 6. The plan calls for the election of army chief General Michel Suleiman as president, the formation of a national unity government in which neither side has veto power, and a new elections law.

Although Syria and Iran say they back the Arab plan, which the ruling coalition has accepted, the opposition is insisting on assuming a third of the ministries in a new government to have veto over key decisions.

Moussa's first visit on Jan. 8 to sell the plan failed and he threatened to walk away from the crisis. However, he is expected to travel to Damascus on Thursday and return to Beirut in the hope of getting all parties to agree to the initiative.

But if he fails again, analysts believe the crisis will be "internationalized," by handing it to the U.N. Security Council -- a concern that Moussa and other diplomats predicted would happen if the conflict was not solved quickly.

Mosali said Tuesday's bombing was "just the latest reminder of how volatile the situation is," and he said he expected it to speed up the "internationalization" of the crisis.

Many Lebanese also fear that the longer it takes for the feuding parties to agree on filling the presidential seat, as a first step toward resolving their differences, the higher the risk for more bombings, as the number of anonymous perpetrators and targets appears to be swelling. - Middle East Times