After three weeks of standoff, broken only by each new shooting, Washington-area police and the serial sniper they are hunting have abruptly moved into an elaborate, high-stakes negotiation.

Montgomery County police Chief Charles A. Moose pleaded with the mysterious shooter last night to contact police directly, promising in a televised address at midnight: "Our word is our bond.

"Let's talk directly," Moose said. "We have an answer for you about your option. We're waiting for you to contact us."

The chief's lengthy, detailed appeal followed three days of guarded back-and-forth and suggested that the process - what some investigators had viewed as a hopeful step - has not been smooth or successful.

Moose indicated last night that authorities had not been able to talk directly with the shooter, who had communicated only via notes, indirect messages or phone calls to other jurisdictions. The inability to have a one-on-one conversation, Moose said, "has been a frustration to us, as it has been to you."

The chief urged the shooter to call a private, toll-free phone number or to write to a secure post office box. But even if the two sides connect, some crisis experts and criminal psychologists say it is unlikely that authorities will build enough trust to win the sniper's surrender or get close enough for a capture.

Unlike hostage or terrorist situations, there is little precedent for negotiating an end to a serial murder case or other unfolding crime. And in the case of the sniper who has eluded police in the Washington region since Oct. 2, there also is little indication in his actions so far that would suggest that he will give up quietly.

A catch-22?

Some experts said yesterday that his recent communications with police could be carefully scripted to advance his own frightening campaign.

"It strikes me that this person doesn't want a dialogue or a negotiation," said Charles Patrick Ewing, a forensic psychologist and law professor at the State University of New York in Buffalo. "He wants to keep his status quo, which is terrorizing the community."

Through his recent messages to police, the sniper may have done nothing more than to create a catch-22 for the authorities, Ewing said.

In a letter left at the site of the Saturday shooting in Ashland, Va., the sniper demanded that $10 million be wired to a domestic bank account and threatened further violence, according to law enforcement sources, who said the demand was repeated in another note found after a shooting Monday at a Montgomery County bus stop. Ewing questioned whether the shooter's main intention was to put police in what he called a "lose-lose situation."

"This is a very smart individual who knows they're not going to give him $10 million," Ewing said. "But if the killings continue, it's like - he's made an offer, they refused it and now more people die. The police are in a horrible spot."

Last night's dispatch suggested that the gunman wanted to put the police through other curious paces as well.

"You have indicated you want us to do and say certain things. You asked us to say, 'We have caught the sniper like a duck in a noose,'" Moose said last night without elaboration. "We understand that hearing us say this is important to you."

Moose and other officials have refused to comment or take questions on any of the messages delivered publicly to the sniper, through the news media.

'One step closer'

Earlier yesterday, one of the lead federal investigators expressed optimism about the case.

"We get one step closer every day," said Michael R. Bouchard, special agent in charge of the Bureau of Alcohol, Tobacco and Firearms' Baltimore division.

"Just like an illness, some things don't respond to treatment right away," Bouchard told reporters yesterday. "Some cases don't respond to our methods right away."

Investigators can use tested negotiation methods to turn even the most unreasonable demands to their benefit, criminal justice professors and former police officers say.

The communications between police and the shooter - some of which have played out on live television as authorities have responded to the sniper's writings and phone messages - could yield critical new clues about the shooter's personality and motives.

"My sense right now is, they're not trying to talk him in," said Mike Aamodt, a psychology professor at Radford University in Virginia who trains police in hostage and crisis negotiations. "That would be great, but I don't think there's any expectation that's what's going on."

Instead, Aamodt said, the ongoing dialogue is feeding two important needs: "The shooter, he has a need for power, and the police have a need for information."

From the notes found at crime scenes, investigators now have handwriting samples and clues about the shooter's educational and social background, motivation and possibly his nationality.

Mad Bomber case

In one of the first instances of modern criminal profiling, investigators tracking the Mad Bomber in the 1950s were left with little more than a series of mysterious, paranoid letters as they tried to solve a string of bombings in New York City that stretched over more than a decade.

Eventually the notes, pieced together in words and letters cut from magazines and newspapers, bore fruit: Investigators determined a pattern of hostility toward the power company, Consolidated Edison, and eventually traced the bombings to a disgruntled former employee, George Metesky.

In the current sniper case, investigators also have recordings of calls reportedly made by the sniper to an FBI tip line. Those phone calls could allow police to study his speech patterns and language for clues to his psychological state.

In some of the calls, the shooter reportedly became irate when he felt he was not given proper respect or attention. The Washington Post reported yesterday that the suspected sniper used such phrases in the phone conversations as, "Hear me out," or, "Just shut up and listen."

A sign of progress

Cecil Pearson, a former hostage team coordinator with a Nevada sheriff's department, said that Montgomery County's Moose has wisely responded in public by calmly inviting the shooter to play a role in ending the bloodshed.

"You indicated that this is about more than violence," Moose said in his fourth and longest televised address to the killer Tuesday evening. "We are waiting to hear from you."

Pearson said that even if Moose appears at times to be letting the shooter call the shots, the dialogue is a signal of progress.

"I think the foundation of trust is being laid by Chief Moose. He's offering all these things, he's trying to get the individual to trust him," Pearson said. "Hey, if the hostage [taker] has chosen the music or the song to dance to, dance to it."