Law enforcement authorities continued yesterday to sift through debris of Wednesday's bomb explosions here but reported no progress in determination what the bombs were made of or who placed them.

"We still have a full-scale investigation underway, but we haven't developed anything of a positive nature as yet," said an FBI spokesman.

Evidence collected at the sites of the two bombings - near the White House and near Soviet offices at 16th and L Streets NW - was being studied yesterday by the FBI and D.C. police.

Investigative sources said analysis of debris from the 16th Street bombing was particularly difficult because the bomb apparently was placed near a metal air duct at the rear of an office building.

Fragments of metal from the damaged duct tend to mask the presence of metal that might have come from the bomb itself, the sources said. Metal fragments from the bomb could indicate that a clockwork timing device was used. Officials have said a timer apparently was used in the blast near the White House on E Street NW.

EPS officers discovered early yesterday what was described as a suspicious package on the doorstep of the Cuban Embassy at 2630 16th ST. NW, which is undergoing renovation.

Two dogs trained to sniff out explosives were shown the package, an EPS spokesman, said, one dog gave a "positive" reaction, and the other "negative," the spokesman said, and the D.C. police bomb unit was summoned. The alert finally ended about 4:30 a.m. when the package was found to contain fried chicken.