WASHINGTON (AP) _ Evidence from the bombing of two U.S. embassies in Africa is to arrive at the FBI laboratory this weekend for tests to confirm what explosives were used and whether both bombs had the same ingredients.

Officials in both Kenya and Tanzania have given permission for evidence to be tested in the United States, Assistant FBI Director Donald Kerr, who heads the FBI lab, said Thursday. Kerr was among top bureau officials who used a briefing here to praise the cooperation the FBI is getting from local officials in Nairobi and Dar es salaam, the East African countries' capitals.

That sharply contrasts with FBI complaints about officials in Saudi Arabia during their joint investigation of the still-unsolved 1996 truck bombing at a U.S. military housing complex near Dhahran, where 19 U.S. airmen were killed. No evidence was sent to the FBI lab from that blast at Khobar Towers.

Meanwhile, President Clinton on Thursday led the nation in mourning 12 Americans killed in the Aug. 7 bombing in Kenya. Standing before black hearses bearing flag-draped caskets in which 10 of the bodies were brought home, Clinton said, ``No matter what it takes, we must find those responsible for these evil acts and see that justice is done.''

A senior U.S. official has said a Czech-made high explosive called Semtex, seen in previous terrorist attacks, is suspected in the nearly simultaneous East Africa bombings, on the basis of initial field examinations. FBI officials would not confirm that.

``Field tests are not 100 percent accurate,'' said Thomas H. Jourdan, a chemist who heads the FBI lab's materials and devices unit. ``They just tell us presumptively what we're looking.''

Assistant FBI Director Thomas Pickard, supervising the investigation from Washington, said 215 FBI agents, lab examiners, evidence technicians, computer specialists, photographers and translators were now in the two East African cities.

A British forensic team was expected to join the 22 FBI lab examiners at the two sites shortly.

``It will take at least four more weeks to complete examination of both bomb sites and witness interviews, and from that we will develop leads,'' Pickard said. There were 700 interviews to conduct in Nairobi and 200 in Dar es Salaam, he said.

Pickard said the FBI does not ``have any plans now to charge anyone'' but noted that in both countries ``local authorities have detained people for various reasons.'' Pickard would not label any of them suspects. But a U.S. law enforcement official said earlier that two non-Kenyans held in Nairobi since the beginning of the week were considered suspects.

The two bomb sites are quite different. The bombed-out center city Nairobi embassy presents the greater difficulty, because the crime scene was contaminated by rescue workers trying to save victims and recover bodies, FBI officials said.

As a result, Jourdan said, evidence technicians are looking for uncontaminated surfaces or shards near the Nairobi blast site. Evidence technicians are wiping these with cotton swabs for microscopic traces of bomb residue invisible to the naked eye.

``We're searching for pure, uncontaminated evidence, like a piece of window glass that was blown across the road and embedded in a wall above the height of people who might pass by it'' or a nearby fence-top surface above the height of a human, Jourdan said.

``What we're after is evidence that was closest to the explosion,'' Kerr said.

To prevent accidental contamination, ``we have to bring back evidence in separate planes from the separate sites,'' Pickard said. Material from each site will then go to separate areas of the FBI lab.