Federal investigators working to determine what caused a single-engine turboprop plane to crash near downtown Leesburg on Saturday afternoon, killing all three people aboard and narrowly missing several homes, said they expect to release a preliminary report next week.

The mangled wreckage of the Socata TBM-700 was loaded onto a flatbed truck Monday afternoon and taken to a facility in Delaware, where it will be examined for possible mechanical problems, according to the National Transportation Safety Board, which investigates airline accidents.

Investigators are also reviewing the pilot's medical records, his communications with air traffic controllers and whether dense fog contributed to the crash.

"We're looking at the usual things you look for in an aviation accident," said NTSB spokesman Terry Williams. "We look at flight patterns. Naturally, we look at the engines. We look at the weather."

Witnesses said they saw the six-seat aircraft bank and descend quickly about 2:45 p.m., shearing branches from several trees before landing within 10 feet of a house between Loudoun and Market streets, authorities said.

Pilot Donald W. Fitzpatrick, 58, of Reston, and co-pilot Gregory D. Jackson, 42, of Sterling, were killed in the crash. Passenger Bronson Byrd, 56, of Purcellville, was taken to Inova Fairfax Hospital, where he died.

The crash, the second fatal plane accident in Leesburg in eight months, has prompted town officials to ask whether they can improve safety at and around Leesburg Executive Airport. The other plane also crashed within feet of a house.

Leesburg Mayor Kristen C. Umstattd said she would ask NTSB investigators and Federal Aviation Administration officials to meet with the Town Council. She also asked for a staff briefing at Monday's council work session about Saturday's crash and one in July. The council might consider "closing the airport during periods of fog" as a precaution, she said, even before the NTSB determines the cause of the most recent crash.

"We don't know for certain yet what caused the crash, so we don't know if there was a remedy," Umstattd said.

Airport Director Doug McNeely said the FAA and an independent contractor have checked navigational aids at the airport, a standard precaution after a crash, and determined that they were working properly. To land in poor visibility, pilots use the localizer, which helps to orient them with the runway, and the automatic weather observation station, which broadcasts weather conditions at the airport.

Given Saturday's fog, pilots would have had to rely on instruments to land because they could not orient themselves by looking out the window, McNeely said.

"With the foggy conditions we were experiencing, it would be up to each individual pilot to decide whether to come in," he said.

McNeely said he could not speculate on the cause of Saturday's crash and did not know whether fog played a role.

The airport has 269 aircraft and a 5,500-foot runway and, like most of its size, does not provide "glide slope" information, McNeely said. A glide slope transmitter indicates the angle that a pilot should take to clear all obstacles before the runway.

Umstattd said residents had suggested changing the airport approach to the south, rather than the current approach from the north, which brings planes over Leesburg. The FAA determines the safest approach to each airport, McNeely said.

Planes approach Leesburg airport from the north because it is just north of Dulles International Airport's air space. Leesburg is a relief airport for Dulles, which means that planes would be diverted there if they could not land at Dulles.

FAA spokesman Jim Peters said it would not be unusual after an incident like Saturday's crash for a town to request that the agency reexamine an airport's approach path.

But federal and local officials said they can do little before they know what caused the crash. NTSB officials said that although the preliminary report is expected next week, the investigation could take months.

"As pilots, the first question we ask ourselves is 'Oh, God, why?,' " said Stephen Axeman, chairman of the Leesburg Airport Commission and an FAA manager. "The only way not to have aircraft accidents is to not fly. The only thing we can do as a commission is to educate people and hopefully learn from it."

The Leesburg Town Council is considering a rezoning application to build more than 300 homes near the north end of the runway.

The plane took off from Sarasota, Fla., and stopped in Greer, S.C., where a passenger was dropped off, authorities said. It was to land in Leesburg at 3 p.m. Peters said radar showed that Fitzpatrick crashed about a mile from the airport while attempting to land.

Neighbors said they heard a deafening roar as the plane passed just above their roofs before plummeting into Realtor Bob Brown's back yard near Market and Ayr streets. The Brown family was not home, but other neighbors rushed to try to help those in the plane.

Loudoun fire and rescue officials, who were called at 2:46 p.m., requested a helicopter to rush Byrd to the hospital, but none could land because of the fog, said spokeswoman Mary Maguire. Byrd was taken to Loudoun Hospital Center's Cornwall campus and then by ambulance to Inova Fairfax Hospital.

Rescue officials advised families in 18 homes to evacuate because of diesel fumes.

Donald Shea, who lives on Market Street, was at a bar mitzvah in Maryland when the plane crashed through his back yard, and he rushed home after hearing the news. Firefighters escorted him into the neighborhood and through his back fence to take his dogs from the house. Like his neighbors, Shea expressed sadness for the men's families but relief that the plane had not struck any houses.

"It just missed the houses by feet," Shea said. "If you look at the density of these homes and the fact that not a house was hit, that's pretty amazing."

Saturday's crash occurred about a mile from where James M. Scambos, a United Airlines pilot who lived in Ashburn, died when he steered his two-seat plane away from the downtown area and crashed in a driveway at the end of a cul-de-sac on July 6. The final NTSB report on that accident is to be released this month.

On Monday afternoon, the diesel odor was still heavy in the air, and branches lay on the drive between Loudoun and Market streets, where the plane had severed a tree trunk. "The scary part is, we're just looking at the tree tops it took off," said Chris Straight, pointing through his blinds to severed branches less than one story above his roof, "and it probably came over this house" or that of his next-door neighbors.

"It's quite scary. It's very scary, especially it being the second one now in Leesburg," said Fran Reddle, who lives near the crash site on Loudoun Street. She arrived home soon after the crash and said she thought that police cruisers and rescue vehicles were there because of a car accident.

"I never ever thought it could be a plane," Reddle said while watching a backhoe load parts of the plane on to a flatbed truck next to the battered fuselage.

Several of the victims' relatives took a last look through the cockpit windows before leaving with a bag of golf clubs recovered at the site.

"We'll never know what they were thinking at the last moment," Straight said, wondering "if they purposefully didn't hit the homes or if they were lucky."

Shea pointed out that his fence, knocked over in two places, "could be repaired with money."

"We were lucky," Straight said, "a lot luckier than the poor guys in the plane."

Fire officials inspect the Socata TBM-700 plane after it crashed behind a house in Leesburg on Saturday.