After a tarot card containing a handwritten message for police was found near a Bowie middle school on the day a 13-year-old boy was shot and critically wounded there, investigators theorized that a deadly sniper roaming the Washington region was trying to open a line of communication with them.

Yesterday -- two weeks after the Bowie shooting, and after the 13th victim attributed to the sniper was critically wounded Saturday night in Ashland, Va. -- investigators said theory had become reality.

They believed that they were engaged in a bizarre dialogue with the elusive gunman. They said they are confident that a written message found at the site of Saturday's shooting was left for them by thesniper, and they have been responding to him through the media, issuing cryptic, carefully crafted statements at news briefings.

Where the strange conversation will lead, and whether it will save lives, no one knows. But law enforcement experts not involved in the case said that if the message left in Ashland is genuine, investigators may have a highly unusual opportunity to forge a relationship with a serial killer blamed for a dozen sniper shootings, nine of them fatal, since Oct. 2.

Echoing other outside experts, Park Dietz, a California-based psychiatrist who has been involved in a number of high-profile criminal cases, said, "It's an unusual thing for serial violent offenders to communicate with law enforcement . . . during their offenses." But it is not without precedent, he said.

And those serial offenders who do leave notes often have different reasons for communicating with the authorities or others.

"Some of them are doing it to taunt police," Dietz said. "Some of them do it so they can get more credit. What I don't take it to mean ever is a desire to be caught. . . . I don't think people capable of serial homicides feel enough guilt."

Experts point out a big distinction between the messages believed to have been authored by the sniper and notes and other writings that were sent to law enforcement authorities by such serial murderers as Theodore J. Kaczynski, the Unabomber; David Berkowitz, known as the Son of Sam; and the unknown assailant nicknamed the Zodiac killer.

"It is usually one-way. Whether it is Kaczynski, the Son of Sam or Zodiac, it is them writing to us without giving us the opportunity to dialogue with them," former FBI profiler Clinton Van Zandt said. "In this case, however, there is a chance to dialogue."

Van Zandt said the rare opportunity could help the investigation.

"Number one, we would like to dissuade him from his behavior," Van Zandt said. "Number two, it allows us to assess both who he is and what he is doing. It will help determine the demographics of the writer or the caller."

Joe Coffey, a former New York City police lieutenant who supervised the Son of Sam investigation, found the one letter that Berkowitz left for police during a 13-month stretch in 1976 and 1977 in which he fatally shot six people and wounded seven. Coffey discovered the letter in the Bronx at the scene of one of the shootings. It was in that letter that Berkowitz -- who subsequently wrote another note to newspaper columnist Jimmy Breslin -- first called himself the Son of Sam.

Coffey said that these forms of communication from a criminal are helpful in developing a profile of the assailant, but also can help prosecutors. In the Berkowitz case, the letter became a crucial piece of evidence during his trial.

"We got a palm print off of the letter, and that helped get him convicted," Coffey said. "It was an exact match."

Coffey said that those serial killers who leave letters for the police generally do so at a point when their criminal sprees have built momentum. "Once they get on a roll, killing a lot of people, they want to make sure that they get credit for it," he said. "It's all about ego."

Law enforcement officials said that communiques from killers -- whether in letters or by telephone -- can provide investigators with glimpses into a criminal's emotional state and motives. Such messages also can help investigators determine an assailant's age or education level.

But much of the analysis could end up being extrapolation or guesswork, authorities said, noting that messages from criminals can be obtusely written or rife with grammatical errors that may or may not be intentional.

Messages from serial killers are hardly a new phenomenon.

In 1888, London's notorious Jack the Ripper, who relished writing boastful letters, taunted Scotland Yard with the written vow: "I am down on whores and shant quit ripping them til I do get buckled."

Eighty-one years later, the Zodiac Killer, who has not been caught, sent a bizarre series of coded letters to police claiming responsibility for 17 murders in Northern California in the 1960s and '70s.

During a string of attacks on women in the early 1980s, Paul Michael Stephani called the Minneapolis police and, in a wailing, weepy voice, pleaded with them to catch him before he killed again. He was then referred to in the media as the "weepy-voiced killer."

And the Son of Sam, Berkowitz, wrote in his letter to Breslin: "Like a spirit roaming the night. Thirsty, hungry, seldom stopping to rest: Anxious to please Sam. I love my work."

Tod W. Burke, who teaches criminal justice at Radford University in Virginia, said the sniper's campaign of random shootings in the Washington region seems like a "grand-scale hostage situation," with authorities attempting to broker a resolution.

Montgomery County Police Chief Charles A. Moose, who has been speaking publicly for a regional law enforcement task force hunting the sniper, "is acting like the negotiator, and he is doing so in a very deliberate and calculated public way," Burke said. "But in most hostage cases, negotiations are private, and the media are observers and not participants."

Burke said he believes that the sniper has been attempting to communicate with police almost from the outset of the attacks, which have occurred in Montgomery, Prince George's, Spotsylvania, Prince William, Fairfax and Hanover counties and in the District.

"The tarot card is a signature and a means of communicating," Burke said. "Maybe all the person wants is for someone to listen, and perhaps he is expressing this through extreme measures -- killing people."

John J. Baeza, another former New York detective, said that authorities handling the sniper investigation "need to assess the cost and benefit" of releasing the gunman's messages, particularly if the handwriting could possibly be recognized by people who know the suspect.

"I strongly urge that they put that out there so that people can identify that," Baeza said. Referring to the remote Montana hovel where the Unabomber lived and was arrested, Baeza said the sniper"is not a ghost. He is not living like Ted Kaczynski somewhere. Somebody knows him."

Former FBI profiler Gregg McCrary said Kaczynski had written letters to the bureau in which he mocked investigators for not being able to capture him. Despite those letters, it was another piece of writing -- the Unabomber's manifesto, jointly published by The Washington Post and the New York Times in 1995 -- that helped FBI agents finally end his 18-year-long campaign of mail bombings. It was Kaczynski's brother, David, who recognized some of the phrases in the manifesto and led agents to his Montana shack.

The Unabomber's "real downfall wasn't his letters to us, but the manifesto," McCrary said. "We wanted it published for the investigative value that somebody would recognize his writings. You can't write a 35,000-word manifesto without revealing something about yourself that someone may recognize."

In Berkowitz's case, it was not his writings but a parking ticket issued to him near the scene of his last killing that led police to him.

The messages believed to have been left by the sniper could contain a key to his identity, according to Eric Hickey, a criminologist who worked with authorities on the Unabomber case and also teaches criminal psychology at California State University at Fresno.

The Unabomber "was sending out messages because he wanted to be heard," Hickey said, adding that the sniper "strikes me as somebody who wants to be heard" as well.

"He's saying, 'I'm out here, and I'm not going away.' " 

Research editor Margot Williams contributed to this report.