A man who claimed to have seen the sniper shoot his most recent victim made up the story, providing false descriptions of a gunman, a weapon and a vehicle, police said yesterday as they discounted what had seemed to be some of their most promising leads in the case.

The witness's account led police to erroneously tell the public to watch for a cream-colored Chevy Astro van with a malfunctioning taillight and a ladder rack. The man also wrongly described the gunman and the rifle he used to kill Linda Franklin, an FBI analyst, as she stood at her car at a Home Depot store in the Seven Corners section of Fairfax County on Monday night.

"Information provided by one of the witnesses at the scene of the shooting in the Home Depot . . . is not credible," said Fairfax Police Chief J. Thomas Manger. "In addition, there have been several media reports related to the description of the specific weapon and the suspect, and that information as well is not reliable."

As detectives checked and rechecked leads because of the false witnesses account, the search for the elusive sniper expanded internationally. FBI interrogators are questioning detainees at the U.S. military detention center in Guantanamo Bay, Cuba, about whether they know of any specific al Qaeda plans to launch sniperattacks in the United States, law enforcement sources said.

Authorities continued to stress that no evidence has emerged that would link the sniper to foreign or domestic terror groups. And a senior defense official said that Army surveillance aircraft that were supposed to assist in the hunt for the sniper had not arrived yesterday.

Montgomery County Police Chief Charles A. Moose, who is heading the investigation into the string of shootings that has killed nine and wounded two since Oct. 2, said the false witness accounts did not set back the investigation.

But yesterday, police cadets returned to the site of Monday's shooting and scoured the grass and pavement across Route 50 from the parking lot where Franklin was killed by a gunshot to the head. Sources said police searched the area Monday night and Tuesday before the rain came, but not as thoroughly as they did yesterday after they learned that the witness had provided false information.

The cadets did not recover a shell casing or other evidence, the sources said, and police now believe the sniper may have fired from about 100 yards away -- more than twice the distance the witness had claimed.

Detectives discovered that the witness's account was wrong when they checked it against versions given by others, said Lt. Amy Lubas of the Fairfax police. Police are now talking to still more witnesses who may have seen a white van fleeing the shooting, sources said.

Asked whether the man intentionally misled investigators, Moose said curtly, "Yes."

Fairfax Commonwealth's Attorney Robert F. Horan Jr. said detectives are working to determine why the man gave incorrect information but said it is "way, way down the list of priorities" as the search for the sniper continues.

"If I decide I can prove this guy deliberately lied to mislead the police, I'll charge him," Horan said. "If I decide he was mistaken or got a little carried away, that's a different story. I'm not conceding he lied. A lie connotes a deliberate attempt to mislead police. He's definitely wrong in his perceptions. We'll look at it and sort it out."

The three-day lull since the sniper last struck is the longest since the attacks began. For the past two weeks, the roaming gunman has been shooting people mainly in the suburbs and exurbs of the Washington area. He has fired a single .223-caliber bullet from a high-velocity rifle at people in the District and in Montgomery, Prince George's, Prince William, Fairfax and Spotsylvania counties.

No one is sure why the sniper may have slowed his pace. "Serial killers, like most people, have other things to do," said James A. Fox, a criminal justice professor at Northeastern University in Boston. "But I wouldn't bet on him being inactive. One thing that's predictable about this guy is he's unpredictable."

Some law enforcement officials said that the false witness accounts had wasted two days of tracking down leads. "We didn't have much to begin with, and now what we thought we knew, we don't," said one investigator.

That caused the probe into Monday's shooting to shift yesterday. While sharpshooters stood guard on the orange roof of the Home Depot parking deck, 67 police cadets lined up shoulder to shoulder to look for evidence in a large swath on both sides of Route 50.

Wearing rubber gloves, dark blue uniforms and caps, the cadets scoured grassy areas and the Willston Center parking lot on Route 50. Tires of cars parked at the center were checked to see whether they might have run over a shell casing or other evidence.

The cadets, from the Fairfax police academy, looked around dumpsters and on the ground, even picking up litter to peer underneath. At times, they got down on their hands and knees for a closer look.

Police said they were trying to determine the direction of the single shot that killed Franklin. The space where her car had been parked was blocked off with orange cones and empty shopping carts.

There are concerns about searching for clues three days after a crime has been committed. Police officers said such factors as the weather and traffic can contaminate the evidence they collect -- or fail to collect.

Law enforcement officials said investigators had no choice but to spend two days following up on the witness's false leads even though some had been skeptical of his story from the start.

"Because of the lies, he caused a lot of work for a lot of people," one law enforcement official said. "But the detectives did some good work to get to the bottom of it, as depressing as that is. It could have gone a lot longer with us chasing bad information."

Police remain confident in their search for a white box truck that witnesses saw speeding away from the Montgomery County shootings, and for a white van with a ladder rack on the roof like that seen near the second Spotsylvania County shooting.

Moose yesterday asked people to "keep an open mind" and phone in tips even if they don't exactly parallel what they have heard.

He said that ultimately, police will track down the sniper, but he added a proviso: "We still need help from the community to make this case."

They still do not have the promised help from the Pentagon. Secretary of Defense Donald H. Rumsfeld signed an order Tuesday allowing aircraft, including RC-7 airborne reconnaissance low planes, to assist in the search. The RC-7 aircraft are loaded with sophisticated sensors and can provide surveillance over a large area day and night. In the event of another sniper attack, telescopic cameras on the planes could be directed toward the site of the shooting and transmit high-resolution imagery to the ground immediately.

In addition, the National Imagery and Mapping Agency, a top-secret spy agency, is providing special electronic area maps and "predictive analysis," sources said.

Investigators in Cuba also have joined the hunt. Osama bin Laden's al Qaeda network is known to have discussed a variety of terror tactics for use against U.S. facilities and political figures, from bombings to the use of poisons. Firearms training and target practice were included among the offerings at al Qaeda camps in Afghanistan, which are believed to have served as training bases for thousands of potential terrorist operatives.

"Kidnaping and Assassinations Using Rifles and Pistols" is one of the chapters in a 180-page al Qaeda terrorist manual obtained by police in Manchester, England, that was later entered into evidence at the New York trial of defendants in the 1998 East Africa embassy bombings.

The document includes information about assault rifles such as the AK-47, plus advice on staking out a victim and targeting key areas of the body, including the head or neck. The manual also suggests using two to four people to successfully carry out an assassination, depending on the type of assault chosen.

All of the scenarios, however, seem intended for victims targeted because of their political affiliation or position. Law enforcement officials point out that the victims of the Washington area sniperhave apparently been chosen at random and have widely different backgrounds. In addition, authorities say they have not discovered any evidence of political or terrorist motives near any of the victims.