In this story:

• Sifting through debris
• ATF to join probe
• Embassy location 'problematic'
• Emotional ceremony marks end of rescue efforts
• Related stories and sites

NAIROBI, Kenya (CNN) -- The FBI and Kenyan police briefed reporters for the first time early Thursday on the investigation into bombings at U.S. embassies in Kenya and Tanzania, saying they have detained about five people for interrogation and that they may have found parts of the vehicle that carried the bomb in Nairobi.

FBI Special Agent Sheila Horan expressed on behalf of the FBI her deep regrets to all those who have "experienced a personal loss or injury as a result of this tragedy."

She pledged "the commitment and resources of the FBI in identifying those who are responsible for this malicious act and to bring them to justice."

"We will pursue this investigation to its conclusion," Horan said, repeating the pledge a second time for emphasis.

In outlining how the FBI would proceed, Horan said the investigation would consist of two phases. She called the first the "investigative phase," consisting of "massive amounts of interviews" with bombing witnesses. Horan said it would take "a very long time to get through those people." But she said the interviews were needed because even a small piece of evidence could be "very valuable to us."

Senior Kenyan police official Peter Mbuvi said the Kenyan police have detained for questioning about five people because of "suspicious activities." He would not indicate the nationality of any of the people detained.

Horan said the other half of their investigation was the "forensic evidence collection phase."

"We are literally sifting through tremendous piles of debris in an effort to come up with that piece of evidence which we can use to complete this investigation," Horan said.

She also said that the investigative teams have "been successful, we think, in identifying certain parts of the delivery vehicle."

At noon on Wednesday, a bulldozer removed what could be a vital piece of evidence: a vehicle plastered against the embassy wall by the force of the blast and which may have carried the explosives.

That vehicle and dozens of other mangled wrecks destroyed in the blast were hauled across the street to a parking lot where FBI investigators have been examining them for clues.

U.S. officials say 175 FBI agents and other investigators are in Kenya and in Tanzania.

The Bureau of Alcohol, Tobacco and Firearms said Thursday it will send a group of top investigators to Africa to supplement the FBI's efforts.

The group of about eight investigators initially will focus on evidence in the bombing of the U.S. embassy in Kenya, but will take specialized equipment for use in gathering evidence at both bomb sites, ATF officials said.

The group includes many who investigated the World Trade Center bombing in New York and the bombing of the federal building in Oklahoma City.

Foreign Service officers undaunted by bombings Mideast, African embassies tighten security after bombings Protecting embassies difficult ... and probably impossible U.S. victims in Friday's bombing in Nairobi, Kenya Chronology of attacks on U.S. targets Image gallery Message board: Africa explosions

On Wednesday, new details emerged indicating that the United States knew more than eight months ahead of the attacks that security in Nairobi was inadequate.

U.S. Ambassador to Kenya Prudence Bushnell warned the State Department, in cables sent in December 1997 and April or May 1998, that security at the Nairobi embassy needed to be upgraded.

In one of the Bushnell cables the ambassador noted that "the location (of the embassy) is problematic" because of its position next to "one of the busiest streets in Nairobi."

In December of last year, Ambassador Bushnell told her supervisors she needed a new embassy, making the point that newer embassies were "built to different specifications from our embassy, which was constructed in the '70s."

An assessment team from the State Department's Bureau of Diplomatic Security made a trip to Nairobi in January and agreed that the location of the embassy was undesirable.

On June 1, Bushnell, who was injured slightly in Friday's terrorist attack, got a written response to her calls for a new embassy from Undersecretary for Management Bonnie Cohen.

"In light of the current threat level and comparative recent construction of the building, a new building was ranked low in relative priority to the needs of other embassies," Cohen wrote.

A new embassy would have cost around $75 million and taken four years to build. According to the government's system of ranking countries by degree of danger, the Nairobi embassy was considered a low threat level and therefore other embassies took higher priority.

At an emotional ceremony Wednesday evening before a symbolic small pyramid of concrete rubble, the rescue officially ended. Those who spent five days trying to save lives honored the dead.

Several wreaths were laid against the concrete pyramid, on which Bushnell scattered roses. And after the ceremony, Bushnell, her lip stitched and hand bandaged from bomb injuries, walked to the barbed wire coil surrounding the embassy holding a single rose. She wept, her shoulders shuddering.

For many, perhaps the heaviest death to bear was that of Rose Wanjiku Irungu, a Kenyan woman whose whispers and pleas from a deep, dark corner of twisted steel and concrete touched the hearts of millions around the world. She was last heard on Monday.

On Wednesday rescuers peeled away a wall of concrete that had prevented her rescue, and her husband's quiet vigil of hope at the site ended. Her death was, for many rescuers, the death of hope.

Kenya remains a grief-stricken nation, and the long-term aid effort to help the victims is just beginning.

The opposition Safina party, headed by paleontologist Richard Leakey, called on the U.S. government to meet its "moral obligation" and pay the medical expenses of those injured and pay for rebuilding.

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