An FBI crew inspects cars destroyed in the Nairobi blast

Latest developments:

• Kenyan, Tanzanian bomb evidence kept separate
• Military offer of security survey turned down
• Review board to look at embassies' security
• Related stories and sites

WASHINGTON (CNN) -- FBI laboratory scientists expect to begin analyzing evidence from both the Nairobi and Dar es Salaam embassy bomb sites by early next week and should have tentative test results within days on the types of explosives used in the blasts.

On Friday, officials said they expect the first two planes carrying evidence from Kenya and Tanzania to arrive in Washington sometime this weekend.

Sources tell CNN preliminary lab screening with portable field equipment suggested the presence of ammonium nitrate and fuel oil. But they emphasize the tentative nature of those tests and say they will make no conclusions until they can analyze the residue with the highly sophisticated laboratory equipment at FBI headquarters.

Lab officials tell CNN that it will take probably "two or three days" to get the first results on the nature of the explosives.

Kenyan, Tanzanian bomb evidence kept separate

Although it is generally assumed the same individuals are responsible for the near-simultaneous bombings in Kenya and Tanzania, the FBI is treating the sites as two entirely separate crime scenes. Investigators from each site will not share specific information on evidence collection until the analysis is complete to avoid any possibility of biased assumptions.

FBI lab director Donald Kerr has insisted the evidence gathered in Kenya and Tanzania be returned to Washington in separate aircraft to ensure there can be no possible mix-up. The evidence will be kept on separate floors at FBI headquarters and analyzed at separate times to keep the materials fully segregated.

U.S. officials say Kenyan and Tanzanian authorities fully support transporting the evidence to Washington.

Military offer of security survey turned down

The State Department has also disclosed that six months before last week's embassy bombings, the head of the U.S. military's Central Command, Marine Gen. Anthony Zinni, told the State Department he was concerned about security at the Nairobi embassy.

Zinni said a planned renovation for the embassy later in the year "would not change the fact the embassy was close to the street in a busy intersection," Assistant Secretary of State Patrick Kennedy said.

"The military also suggested that they would do a survey of the embassy, embassy security, I guess," Kennedy said.

The State Department turned down that offer because it was already conducting its own security reassessment of the embassy, in response to similar concerns expressed by Ambassador Prudence Bushnell, according to State Department spokesman James Foley.

A senior State Department official told CNN, "We did not turn down General Zinni's offer because we were not interested. It was because we already had an assessment team going. In general, if a senior military official says he sees a problem area, we immediately get on it. In this case, we already knew of the problem."

Review board to look at embassies' security

The State Department will create a review board to work with the FBI and diplomatic security officials to determine whether inadequate security was to blame for the African embassy bombings.

Under a law passed in 1986, an "accountability review board" must be impaneled "in the case of serious injury, loss of life or significant destruction of property at or related to a U.S. government mission abroad."

The board will be made up of five "eminent" people -- four appointed by the secretary of state and one by the CIA director.

According to a document provided to CNN by the State Department, the board is required to produce written findings to determine four things:

• the extent to which the incident was security-related.

• whether the security systems and procedures were adequate and properly implemented.

• what effect the available intelligence and information had.

• whether the facts and circumstances may be relevant to appropriate security management at posts abroad.

Secretary of State Madeleine Albright will have 90 days after she receives this report to tell Congress her response to the panel's recommendations.