Putting together the pieces of the Beirut embassy blast

The hulk of the six-story building in east Beirut's Awkar suburb stands eerily empty, window frames askew, doors ripped off completely, the front facade blackened from small fires.

One of the few pieces still in place is a white video camera on the roof, installed to monitor movement around the United States Embassy annex.

The camera represents just one of many reasons a suicide bomber was able to get so close to the tightly guarded facility last Thursday. The electronic equipment - including a specially designed gate - had not been installed or fully activated when the American diplomats moved in in July.

Despite the tragedies last year at the original US Embassy and at the Marine compound, Western military and diplomatic sources suggest that the Americans did not take sufficient precautions to prevent a repeat.

On Sunday, President Reagan himself acknowledged that security measures were not completed when the embassy personnel moved in. But, he said to reporters in New York, ''We moved in because it represented more safety than the one we were in (in west Beirut).''

It appears that a British royal military policeman, who was in front of the annex only because his ambassador was paying a courtesy call, was virtually single-handedly responsible for preventing ''a catastrophe five times as bad,'' a US source said.

The British bodyguard fired five shots and hit the driver of the light-colored station wagon as it approached the embassy annex, he recounted later. ''I saw him fall over. As he fell over, he pulled the steering wheel to the right. The vehicle slid sideways into an American van at the side (of the road).''

At least one other Lebanese guard fired at the car but apparently missed, according to the bodyguard, who cannot be identified because of the possibility of reprisals against him. Another Lebanese tried, but his US-made M-16 rifle jammed, US sources said.

Although investigators are still in the beginning stages of a full reconstruction, it appears that none of the American guards inside the building had sufficient warning to fire.

''Warning'' is the key word in this third attack on an official US facility in Beirut since April 1983. Islamic Jihad, an invisible force which claimed responsibility for all three attacks, had issued repeated threats, as recently as five days before the blast.

Assistant Secretary of State Richard Murphy, who arrived in Israel on Monday after talks in Damascus, says the US had been ''on maximum warning for a long time.''

Yet in a simple but brilliant scheme, now so familiar, a lone terrorist was able to penetrate the US grounds with at least half a ton of explosives once again. On Monday, Islamic Jihad reportedly threatened to attack another American target in retaliation for a massacre of villagers in south Lebanon by the Israeli-backed militia there.

Although last week's attack was the smallest of the three in terms of casualties, it is likely to become the most controversial because of the lessons from the past that appear not to have been learned. There is already talk in diplomatic circles about a witch hunt to help cover the embarrassment of a superpower not being able to protect its envoys.

The compound has been sealed off since shortly after the bombing, so tightly secured that one longtime Middle East observer has called it the ''safest place in town.'' But the question remained: Why could it not have been so safe before?

During Mr. Murphy's visit to Beirut he reaffirmed the US commitment to Lebanon and to keeping an embassy open. But as investigators sorted through the debris and staff searched for scattered classified documents over the weekend, there were no solid answers about how the US could operate in the long-troubled state.

Most diplomats are now working out of the crowded residence of Ambassador Reginald Bartholomew, who was injured in the bombing.

Last weekend he told reporters, ''There is no such thing as 100 percent security and still (being) able to do what is important for the US.''

The USS Shreveport and its escort ships, fresh from searching for mines in the Red Sea, are now anchored just off Lebanese waters to offer support services to the Americans investigators on shore. On board is the 22nd Marine Amphibious Unit, the same unit blown up in the bombing 11 months ago.

But a heightened US presence in the area only provides additional targets. When the US Sixth Fleet was offshore before, there were repeated warnings about suicide pilots flying bomb-laden planes or suicide frogmen trying to target the ships.

The sense of fear in Beirut is almost visible. The British do not want to be seen as having foiled the bomber, fearing their embassy or annex will be next. The news agency that received the call from Islamic Jihad would not provide details about the voice for fear of reprisals.

Several American reporters who have apartments in the same building are considering moving for fear it might become a target. On the day of the bombing, Islamic Jihad cautioned ''our Lebanese brothers and all citizens to stay away from American institutions and gathering points'' because of future attacks.