RICHMOND, Va. - Desperate to continue a cryptic dialogue they've established with the serial sniper stalking the Washington region, police urged the killer yesterday to contact them again - specifically, in response to a threatening letter he wrote demanding a large sum of money, law enforcement sources told The Sun.

The letter, found at the scene of the sniper's most recent shooting, is one of the few clues left for investigators, who swooped down yesterday on two gas stations just outside Richmond in a vain effort to capture the elusive gunman. Although police were hopeful the killer had been at that location, their only arrests were of two men who later proved to have nothing to do with the shootings.

The sealed, multiple-page letter was left in the woods behind the Ponderosa restaurant in Ashland, Va., where a 37-year-old man was shot Saturday night in an incident authorities linked yesterday to the sniper. The note included a phone number for police to call to discuss the letter's demands, law enforcement officials said last night.

In the hostile letter, the killer threatened extreme violence. He also demanded money, officials said.

But by the time investigators had found and processed the letter, the appointed time for the call had passed, said the officials, who spoke on condition of anonymity.

Montgomery County Police Chief Charles A. Moose used a hastily called televised news briefing Sunday evening to urge the author of the letter to call the number given in the note. Investigators routed the phone number to a police location so that when the killer dialed the number it would ring on a detective's desk, officials said.

Yesterday morning, officials said, a 30-second phone call came to police headquarters: a voice so garbled that police couldn't understand it. They later surmised that the voice was a tape recording, officials said.

Police traced the call and raced to an Exxon station near Richmond, 10 miles south of Saturday night's shooting, sources said. There, at 8:30 a.m., three heavily armed officers rushed a white Plymouth Voyager van parked beside one of the station's pay phones and arrested the man inside.

Shortly after driving him away in a cruiser, police arrested another man talking on the phone at the Citgo station across the street.

About 10 a.m., Moose appeared briefly on television to respond to the sniper's communications. "We are going to respond to a message that we have received," he said. "We will respond later. We are preparing our response at this time."

The arrests at first looked very promising: Witnesses of the sniper's previous shootings have described seeing white vans fleeing the scenes, although vans of different makes from the Voyager.

But by midafternoon, investigators had determined that the two men - a 25-year-old Mexican and a 35-year-old Hispanic, both undocumented workers - had nothing to do with the shootings and turned them over to immigration authorities.

The fruitless arrests left police in roughly the same position they'd been in at the day's start: trying to establish contact with the sniper. At a late afternoon news briefing in Rockville, Md., Moose made another plea directed at the person who had called earlier in the day:

"The person you called could not hear everything you said. The audio was unclear, and we want to get it right," Moose said. "Call us back so we can clearly understand."

The intense effort by police to establish clear communications with the sniper signaled a major shift in the investigation into the string of shootings that has killed nine, seriously injured three and gripped Maryland, Virginia and Washington with fear for nearly three weeks.

Two weeks ago, when police received a message from the sniper in the form of a tarot card with "I am God" scribbled on it, at the scene of the shooting of a 13-year-old boy at a Bowie, Md., middle school, they made no attempt to respond explicitly to that message by television.

After the Saturday letter, however, police have apparently decided they must respond directly over the airwaves. In seeking such an exchange, the ante has been upped significantly, said Delores Craig-Moreland, a criminal justice professor at Wichita State University.

"The whole conversation is dangerous, in that you don't know the meaning he will take from the things you say," Craig-Moreland said.

In a reflection of the heightened tension, schools in Richmond and four surrounding districts were closed yesterday - a more drastic step than was taken in the Washington area, where most schools stayed open but in "lockdown" mode. Eleven Richmond area school districts will be closed again today.