Kenyan and U.S. investigators said Thursday they are questioning five people in the bombing of the U.S. Embassy in Nairobi and have made progress in identifying the vehicle that delivered the bomb.

The detainees were held because of "suspicious activities," including one seen by an American at the embassy after the bombing, said Kenyan Deputy Police Commissioner Peter Mbuvi, without elaborating.Mbuvi added that it was too early to say if any of them would be arrested. He would not say exactly how many people had been detained.

A U.S. law enforcement source in Washington said two of them were considered suspects. They were from a country other than Kenya, but their nationalities could not be determined, the source said.

FBI special agent Sheila Horan said the FBI has had some success "in identifying parts of the vehicle" containing the bomb that exploded in Nairobi on Friday. "We are literally sifting through tremendous piles of debris," she said.

Horan refused to comment on reports that the U.S. Embassy in neighboring Kampala, Uganda, had received a warning of an attack two weeks before the nearly simultaneous bombings at the U.S. embassies in Nairobi and Dar es Salaam, Tanzania, that killed 257 people and injured more than 5,500.

In Washington, the State Department disclosed Wednesday that the U.S. ambassador to Kenya, Prudence Bushnell, had said the embassy wasn't adequately protected against an attack and should be moved, but the government did not have enough money to respond.

Patrick Kennedy, assistant secretary for administration, said, "Unfortunately, we simply lack the money to respond immediately to all the needs of embassy construction. We did the very best we could, given what we had."