The Lebanese group the United States is now holding responsible for the TWA hijacking, Amal, is little more than a loosely knit umbrella movement of often-competing Shiite factions and chiefs. Its political leader, Nabih Berri, presides over Amal with uncertain authority -- even over some of his own lieutenants.

At least two of Berri's closest associates have been linked by U.S. intelligence sources to the Trans World Airlines hijacking. But it is still unclear what connection the 46-year-old Shiite leader, holder of an American alien green card, or his organization had to the hijacking.

U.S. government efforts to determine responsibility for last Friday's hijacking have been stymied by the shadowy, highly fragmented state of Shiite politics, its welter of competing religious and secular leaders and a longstanding lack of good U.S. intelligence about that community.

Reaching an assessment has also been complicated by the total chaos in Lebanon today, and by Berri's double role as minister of justice in a nonfunctioning government and shaky leader of the main Shiite militia group.

Another confusing factor is mounting evidence that at least two groups, Amal and the more fundamentalist Hezbollah, or Party of God, are each holding some of the American hostages after both groups sent commandos aboard the TWA plane Saturday during its second stop in Beirut.

Amal and Hezbollah, both relatively new groups, have alternatively been military allies and political rivals for influence among the downtrodden Shiites, heretofore Lebanon's lowest class but now emerging possibly as its dominant political force. Many Shiite radicals appear to belong loosely to both, switching allegiance as the occasion warrants.

Complicating intelligence assessments is their practice of using special names when conducting terrorist operations -- names like Islamic Jihad, Soldiers of God and the Hussein Suicide Squad -- to mask their identity and affiliation.

Perhaps the frankest public assessment of what the United States knows, and doesn't know, about the hijacking was given Wednesday when Assistant Secretary of State Richard W. Murphy testifed before the House Foreign Affairs subcommittee on Europe and the Middle East.

Asked about the U.S. view on Berri's involvement, Murphy replied: "It's still not completely clear what his role was at the beginning of the hijacking."

Some U.S. analysts say there are intelligence reports that Berri had at least foreknowledge of the hijacking, but others doubt that he did, pointing to his initial efforts to stop the plane from landing in Beirut.

Despite a stepped-up U.S. intelligence-gathering effort, Murphy said it had not yet been possible to determine "who pushed the botton, who is the master organizer and where he is at this moment."

"Even the Amal organization is more than one group. Responsibility and chain of command are just not there," he said.

In any case, the United States was still not ready to blame Lebanon, Syria or Iran for sponsoring an act of "state terrorism."

"In Lebanon today," Murphy said, "it is certainly not clear a state can be held accountable for the hijacking. We're sifting the evidence."

For many U.S. analysts, the degree to which Berri or his Amal were behind the hijacking is just as unclear. The Shiite leader at first refused to have anything to do with the hijackers but on the second day switched strategies and sent reinforcement on board the aircraft.

The problem in reaching a judgment about the degree of Amal's complicity is complicated by its past record of involvement in other hijackings and by its loosely organized character -- really a coalition of semi-independent warlords and religious leaders -- that makes it nearly impossible to determine what is done at local initiative and what at the orders of the central leadership.

"It's one of those movements led from the bottom rather than the top," a military analyst said.

Two top Berri aides, one a past master of hijackings, have been mentioned by U.S. analysts as being involved, if not for certain in masterminding the TWA seizure, at least in taking over the operation on behalf of Amal once the plane came to Beirut.

The first is Sheik Hassan Massri, who showed up in the airport control tower to talk to the two original hijackers the first time the plane landed in Beirut. Massri was described by one military analyst as a chief Amal strategist, Berri's "right-hand man" and the director of information for the organization.

The second is Hamza Akl Hamieh, better known locally simply as Hamza, who is the real military leader of all Amal's militia forces in the southern suburbs of Beirut. He has been mentioned by some U.S. officials as the possible organizer of the hijacking.

What makes this plausible is that Hamza, with official Amal blessing, has carried out six previous hijackings, all of them aimed at Libya, to highlight the disappearance there in 1978 of Amal's founder and spiritual leader, the Imam Musa Sadr. The Lebanese Shiites hold Libyan leader Muammar Qaddafi personally responsible for his fate.

Whatever the initial degree of Amal's involvement in the hijacking, it has now taken full responsibility for most of the hostages, thus making itself a clear potential target for any future U.S. retaliation.