On a foggy October evening in 2002, pilot Thomas J. Preziose took off in a small Cessna airplane from the Mobile, Ala., airport on a routine flight carrying express mail. Preziose, an experienced pilot, made the flight to Montgomery dozens of times for a small freight company. He knew the aircraft so well that he was certified to give flight instruction in it.

Six minutes into the flight Preziose urgently told air traffic controllers: "I needed to deviate, I needed to deviate, I needed to deviate, I needed . . ."

The plane was later found in pieces in a swamp. Preziose had died instantly.

Small-plane crashes usually don't get much attention. But as investigators pulled the pieces of the Cessna out of the swamp, they found unusual evidence that led them to report that the aircraft had "collided in-flight with an unknown object at 3,000 feet." Twenty months after the crash, however, the agency still has not found an object.

In an unusual action, the National Transportation Safety Board's Washington headquarters has taken over the investigation, which had been conducted by the agency's Atlanta regional office. The NTSB released an interim report yesterday that makes no mention of a possible in-flight collision.

The NTSB now says that the initial report should not have suggested an in-flight collision. "That sentence should not have been in the factual report," said Ted Lopatkiewicz, a spokesman.

NTSB investigators have now begun to focus on whether the pilot somehow lost control of the aircraft, sending it plummeting into the water. The shift in view is causing some involved in the investigation to raise concerns that the agency is not placing enough emphasis on leads pointing to impact from an object, possibly a military drone.

What has stumped investigators are 34 red marks found on the aircraft's wreckage when it was pulled out of a shallow marsh at Big Bateau Bay, several miles from the Mobile airport. The red smudges and streaks were on pieces of the plane from the nose to the tail, both inside and outside the aircraft. Many were concentrated on the left side of the plane near the pilot's door. The direction of streaks did not show a consistent pattern.

Proponents of the in-flight collision theory say the red marks support their contention. But the NTSB now says those streaks could have come from inside the aircraft. "We have to look for the more obvious sources" first, Lopatkiewicz said. If no match is found, the agency will pursue other possible sources of the red marks, he said.

Investigators say they plan to review a piece of the wreckage recovered last week. The plane's third propeller blade was found buried in the mud near the crash site. It was folded in half with gouges and red marks on its face.

Initial speculation centered on the possibility that the Cessna could have been hit by another aircraft, such as a plane that was not equipped with a transponder. Another early idea explored by the NTSB was that the red marks could have come from an unmanned aerial drone. Many such drones are painted bright red or orange.

Tyndall Air Force Base in Florida, more than 170 miles from Mobile, flies unmanned aerial drones and has had three drone accidents since 2001, according to military accident records and local media reports. No one was hurt in those incidents, although in February a drone crashed near an interstate highway, closing it for several hours.

An Air Force spokesman said no drones were operating on the evening of Oct. 23, and that the drones fly only in military airspace.

The NTSB tested drone material supplied by the military and compared it with the red marks on the plane. It also tested other red items found at the wreckage site, such as red cargo bags. The tests did not yield a match.

"No way we could let that go . . . that we possibly had an unknown midair," said John C. Clark, director of aviation safety at the NTSB. "We can't let it go at that unless we just know we had exhausted every avenue."

Clark said the new probe conducted by the Washington headquarters has led investigators to identify 20 possible sources inside the aircraft that could have caused the red marks. The potential sources, which are being sent to a lab for testing, include a red tow bar, a fire extinguisher and a red baseball cap. Clark said analysis of the markings and gouges on the plane are more consistent with the plane hitting the water at a high speed rather than an in-flight collision.

Clark said the agency does not plan to test any more aerial drones for comparison with the red marks. First he wants to examine the possible sources on the plane.

"When you have a collision like that, there's a violence to them," Clark said. "You don't just wipe across a bunch of parts without putting a bunch of substantial dents or marks [on the plane] and we haven't found that."

Also, he said the Washington team has concluded that a piece of aluminum found embedded in the left wing came from the plane's own instruments in the cockpit. The April report had determined that the piece was not part of the aircraft. The Washington team is also playing down the possibility that the Cessna got caught in the wake turbulence of a nearby DC-10.

Investigators are now exploring whether the pilot might have been confused or experienced vertigo during the flight and lost control of the aircraft.

The move away from the in-flight collision theory by the NTSB headquarters has not convinced all investigators and parties involved in the probe, including Atlanta-based Butch Wilson, who was assigned to lead the NTSB's investigation from Mobile, sources involved in the probe said. Wilson believes the evidence points to an in-flight collision, those sources said.

"You can analyze it 20 different ways, but it's not going to change the fact that ordinarily, you do not have red streaks all over an airplane," said one source close to the investigation.

Wilson said he could not discuss the case without approval from NTSB officials in Washington, who declined to make him available.

The absence of a solution 20 months after the accident has been hard for Moira Wade, Preziose's sister. Wade and her husband, Steve, are both commercial airline pilots. She has become involved in the accident investigation. She has visited the crash site several times looking for wreckage and last week she found the third propeller blade. She believes the blade's red marks and severely bent shape point to a collision.

Wade is convinced that Preziose did not cause the crash and his last words -- "I needed to deviate" -- echo in her mind.

"My brother knew there was no flight data recorder, he knew there was no cockpit voice recorder. He knew there would be no record of him unless he said something," Wade said. "He was trying to tell us something was happening up there and he wasn't able to get away from it. I know my brother and he had only seconds to get that out. And so when I hear that it makes perfect sense to me because I know the guy."

Wade said the unresolved nature of her brother's crash bothers her on both a personal and professional level. "Every time there's an aircraft accident, we learn something. We learn and we train and we learn new procedures and that means all the pilots in the air and all people on the ground are safer," Wade said. "If we're not even going to learn anything from this accident, his death is a waste."