In the weeks before this September's terrorist bombing of the American Embassy annex in Beirut, the U.S. government had specific, reliable intelligence warnings that explosives had been shipped into Lebanon and were targeted against American Embassy personnel, according to informed intelligence sources.

U.S. and Israeli intelligence first tracked explosives and timed fuse bombs in mid-August. Days before the Sept. 20 bombing, they learned that the explosives were designated for use against Americans. A vulnerability assessment narrowed the possible points of attack to two facilities in east Beirut: the ambassador's residence in the southern hills, and the eventual target, the embassy annex to the north near the Mediterranean coast.

Reagan administration officials who have reviewed the intelligence and the details of the attack have found that the failure to take more aggressive security precautions was even more unsatisfactory than first reported. One official called it "inexcusable" and another "negligent." Sources said that too many U.S. security forces were deployed away from the annex, where the most American personnel were stationed.

The analysis of security after the bombing shows that terrorists on Sept. 20 could have had unimpeded access to the embassy from a side road that ran about 200 feet from the annex. No barricades were in place there. Fortunately, the sources said, the driver of the van carrying the explosives approached the building through the front gate and was slowed down along that route by security guards and by concrete barriers around which he had to weave.

The explosives were detonated 30 feet before the van reached the annex, making the damage and death toll less than they might have been.

Sources said the new information about security lapses accounts in part for Secretary of State George P. Shultz's directive last week that he receive a daily briefing on embassy safety as part of a "full-court press" on new security measures.

In addition, the sources said intelligence reports show that some of the explosives are still in Lebanon and another attack is anticipated before the American presidential election. U.S. authorities, according to one source, most fear another attack against precisely the same target.

Under new security measures, all vehicles other than the ambassador's automobile entering the embassy annex in Beirut are stopped. Passengers and deliveries are then loaded into a shuttle service that runs from the gates to the annex, these officials said.

U.S. intelligence, working with friendly intelligence services including those of Israel and Lebanon, have traced the financing of the explosives to an elusive financial middleman with close ties to Iranians who have supported terrorism in the past.

The middleman is identified as Hassan Hamiz, a Lebanese with high-level contacts in the Iranian government. Hamiz was paid $50,000 as part of the operation supporting the October 1983 bombing at Marine headquarters in Beirut that killed 241, according to intelligence reports.

Hamiz is closely associated with Hussein Musawi, a leader of one faction of Shiite militants in Lebanon's Bekaa Valley. Musawi's cousin, Abu Haydar Musawi, was involved in obtaining the pickup truck used in the 1983 Marine bombing, according to intelligence reports. He heads his cousin's group called "Hussein suicide commandos," the reports say.

Intelligence has also established the identity of the driver of the van that carried the explosives in the most recent bombing. The driver apparently had two or three aliases, but officials said he has been traced to the militant Shiite movement called Hezballah, or Party of God, which previously has been identified as the group responsible for the terrorist attack.

The group is a loose confederation. Sources this week cautioned that intelligence data, though concrete and believed to be reliable, is not strong enough to make a case in court.

Last January the Israelis arrested 12 terrorists, including some members of the Party of God, and obtained documents, money, operational structures and target information. But it is difficult for outsiders to get good information from the militant Shiite movements, particularly in advance. "It's like penetrating the top echelon of the Mafia," one source said.

This uncertainty, according to sources, is a major reason the Reagan administration has decided not to retaliate.

The sources said the CIA is more interested in learning about the terrorists, their financing and communication, than in striking back. Though the CIA learned of the explosives shipment and the possible targets, officials said they did not know the timing of a possible attack. "We didn't know when -- whether it was going to be that week or month," one source said. Another official said intelligence warnings without the time element can lose impact, creating what one official called "the cry-wolf problem."

Sources in several western intelligence agencies said this week that Syrian intelligence officers have not been implicated in the latest attack, unlike the 1983 Marine bombing and the April 1983 bombing of the U.S. Embassy in Lebanon.

Among the 13 individuals tied to those two earlier bombings were a Syrian intelligence colonel, a former PLO security officer and others belonging to the Syrian-controlled Thunderbolt terrorist organization.

The absence of Syrian intelligence assistance, according to one source, may in part account for the comparatively low death toll of last month's bombing, in which two Americans and at least 10 Lebanese were killed.

The CIA is continuing its investigation to learn more about those responsible for last month's bombing. The capacity to undertake such a probe has been enhanced significantly since the 1983 incidents. An information exchange network has been set up with the intelligence services, police and military of more than 100 countries.