Investigators said yesterday that they still had not recovered any wreckage that had clearly tested positive for bomb residue.

A law enforcement official said last night that F.B.I. agents wanted to test each piece of baggage retrieved from the area for bomb residue.

Investigators have closely scrutinized the burned portions of the right wing and have tested it for bomb residue.

Other investigators, meanwhile, are subjecting some pieces of the wreckage to sophisticated tests, in a search for bomb residue.

There was even despair on the first, worst days when the highly expensive and sophisticated Egis bomb detection machines sat idle because the available wreckage was so unlikely a source of bomb residue.

The machine that detects preliminary traces of bomb residue had its first positive hit - for PETN, a component of plastic explosives.

The 1.5 billion bags checked each year by passengers will be put through these machines, or screened by a different system for traces of bomb residue.

The bomb residue was lousy with tritium.

And even after more than 2,500 dives, the divers have not yet found material with bomb residue.

Complicating the search for bomb residue is the simple fact that bombs deployed by terrorists in recent years have contained only small amounts of explosives.