1.   Another federal official said Thursday that bomb experts had several reasons to be cautious about the positive finding of explosive residue.

2.   But the scientists in the Washington lab could not confirm that the nose gear had explosive residue.

3.   It should be relatively easy for investigators to determine whether there is any explosive residue in the engine, which landed on a boat parked in a driveway.

4.   One reason is a test result for explosive residue from a part of the right wing, near where the wing and the fuselage are joined.

5.   SFO spokesman Ron Wilson said the guard failed to keep tabs on the shoes or the passenger after the explosive-detecting machine found the explosive residue.

6.   The guard members have learned procedures for passenger screening, including the use of metal detectors and wands that indicate explosive residue.

7.   Travel cases that were potentially contaminated with explosive residue were placed in an area where bomb debris had been stored, rendering them useless for testing.

8.   And bomb experts found no explosive residue, testified William Perry, a supervisory special agent for the FBI in Pittsburgh.

9.   But he said a team of investigators never found any bomb parts or explosive residue, making it impossible to determine from physical evidence how the blast was detonated.

10.   Dogs trained to detect explosive residue were sniffing the ruins of the building, Iliano said.