The FBI is vigorously investigating the crash of EgyptAir Flight 990 even though the National Transportation Safety Board continues to have the lead role in the probe, according to federal law enforcement officials.

Hundreds of federal agents and other experts in Washington, Cairo and elsewhere are working closely with the safety board and Egyptian officials to understand more about the airplane, the crew, the passengers and those who came into contact with the aircraft as it made its journey from Los Angeles to New York.

The other side of the investigation--whether some so-far hidden mechanical failure brought down the Boeing 767--is necessarily in a go-slow mode for now. Safety board investigators can pore over maintenance records and further refine data from on-board recorders, but almost all the physical evidence is buried under 250 feet of water and silt at the bottom of the Atlantic Ocean, where the plane made its deadly plunge about 60 miles off the coast of Massachusetts.

Months and millions of dollars probably will be required to bring up enough wreckage to confirm or overturn strong preliminary evidence from the flight data recorder and cockpit voice recorder that indicates nothing mechanically wrong with the plane. Although they will look, investigators privately doubt they will find what they sometimes call the "eureka piece" that would turn the investigation around.

Meanwhile, the logical course for the probe is to look into the backgrounds, records and activities of the humans involved with the plane--something the FBI is well-trained to do.

Given the strong Egyptian government objections that were lodged when the NTSB prepared to transfer jurisdiction for the Oct. 31 crash to the FBI, it actually benefits the bureau to have the NTSB remain in the lead role, at least for now, officials said. The reason: Egyptian protestations over the bureau's turning the probe into a criminal investigation could have blocked the detail-oriented background work that needs to be done in Egypt, U.S. law enforcement officials said.

"Typically the bureau wants to elbow their way into the lead position, but they are being real sensible," one U.S. official said of the FBI. "It is an institutionally mature reaction to not technically have the lead in order that they might be better able to do their work."

In Egypt, investigations into the backgrounds of all of the pilots and crew are being undertaken by Egyptian authorities at the FBI's request, officials said. In Washington, officials from the NTSB, the FBI and Egypt are working on completing an agreed-upon transcript of the flight's voice recorder.

"People need to understand this is not at a standstill. We are doing everything that we need to do in this case," said FBI spokesman Jim Davis. "The titular head of this investigation . . . really has no impact at all on the work that agents are doing in the field. The interviews are being conducted, the evidence is being reviewed and we have got agents working with the Egyptian authorities."

Having the safety board remain in charge has another benefit: a worldwide reputation for technical competence and honesty. And with certain glaring exceptions--including a bitter dispute with the French government over what brought down a French-built airliner in Indiana in 1994--the safety board is also viewed as fair.

Law enforcement officials said their focus remains on co-pilot Gameel Batouti, who they believe entered the cockpit sometime after takeoff and intentionally pushed the plane into a dive while the captain was absent.

That judgment is based heavily on a series of suspicious actions by Batouti, officials said, including switching off the plane's autopilot after apparently saying a prayer repeatedly.

While his prayer in Arabic--"I have put my faith in God's hands"--has played a role in the probe, it is less significant, officials said, than the actions he took in the cockpit. What remains under intense review, law enforcement officials said, is why Batouti, the father of five and a longtime pilot, might have deliberately taken the actions that led to his death and those of 216 others on board the flight.

So far, the case against him is strong but circumstantial. While public attention has been focused on the prayer, investigators point out that it is important to note what he didn't say. There was no shocked expletive or puzzled comment, as pilots typically make in an emergency or when highly computerized aircraft suddenly take action without pilot input.

U.S. investigators said they were told that Batouti was not scheduled to be in the cockpit until later in the flight, another in the series of factors that has led to a focus on his actions.

Batouti was alone in the cockpit when the autopilot disconnected, an action that will trigger an alarm unless a pilot clicks twice on the shutoff button. There was no alarm.

Eight seconds later, engine power was cut back, and devices on the plane's tail that regulate altitude--elevators--pushed the plane into a precipitous dive. Unfortunately for investigators, the flight data recorder does not record whether elevator controls in the cockpit were moved deliberately. But neither the voice recorder nor data recorder contains anything indicating a malfunction that could cause a rapid elevator deflection.

With the plane at zero gravity, the captain clawed his way back into the cockpit. His comments are inconclusive as to whether he thought something mechanical was happening or suspected his experienced co-pilot of deliberately downing the plane.

Deep into the dive, the elevators split, something that normally happens only when one set of controls is jammed and the other pilot jerks his controls free with at least 50 pounds of pressure. The same thing would happen, however, if the pilots were pulling and pushing in opposite directions.

A few seconds later, someone cut the engines off, an action that requires two deliberate steps and can't happen accidentally. There is no logical reason to cut off the engine on an aircraft that someone still wants to fly.

Sources say that at about the same time the engines shut down, the engine throttles were pushed forward as if someone were trying to increase engine power. It is possible someone or something fell into the throttles, but it is also possible an effort to power up the engines was thwarted by someone shutting down the engines.

None of this is conclusive proof that Batouti downed the plane. And even if he did, it says nothing about a possible motive that could range from the onset of a seizure to suicide to some sort of panic reaction to a real or perceived problem. But it does lead investigators to conclude that he and his life should be closely reviewed as part of the probe.

Davis emphasized that the FBI is looking at a broad variety of possible explanations for the crash and is not focusing on any single individual or action or group. The passengers included dozens of Egyptian military officials. There has been speculation, thus far unfounded, that they could have been a target.

"We are in no rush to make any judgments or come to any conclusions. We very honestly are not putting all our eggs in any single basket," Davis said. "We still continue to look at other possible explanations."

Davis also said the cooperation of Egyptian officials is crucial since the FBI must rely upon Egyptian authorities to obtain records and question people on Egyptian soil about the crew and the passengers.

"We are not allowed to conduct investigations overseas," Davis said. "When you talk about looking into the background of the pilots and crew members and the passengers, any of that work that has to be done in Egypt is conducted by the local authorities. That is their investigation. We can only request that they conduct the investigation."