It was a frighteningly familiar scenario: Five minutes after Fairfax County resident Soroya Petrulis managed to catch up with her runaway beagle pup Sammy, he was convulsing. Within 15 minutes the puppy was dead.

The next day, neighbors watched as a German shepherd died in agony in the street, not far from a gutter where they had just found the body of a neighborhood cat.

Petrulis and her neighbors are becoming used to such incidents. Altogether, 11 cats and dogs have mysteriously died in the past year in the 5800 block of Bush Hill Drive, which is just south of the Beltway near Alexandria.. In one three-day period this month, four dogs and cats died.

No one has any doubt as to what is the cause. It is a clear-cut case of poisoning, with the most likely agent being strychnine, veterinarians, neighbors and county police say. But while the pet poisonings are sickening enough to those who live on the block, what concerns them the most is the implicit threat posed to children, particularly on Halloween.

"This is not accidental," resident Kim Tigner said last week. "We all thought, 'Well, we're running out of animals, so maybe whoever is responsible will stop and start on the kids.' "

Tigner's cat Lewis was one of the first victims. Last October, she said, Lewis slipped out the door when she was bringing in groceries. Within minutes, Lewis was back again, howling and rigid with convulsions.

"It took him 15 minutes to die," she said.

To date, the poisonings have occurred only in the fall -- seven animals last year and four this month, said Detective James M. Flood of the Fairfax County police department.

Initially, one veterinarian who examined a poisoned dog last year told neighbors he suspected the animals were eating antifreeze. But Flood is skeptical.

"They would have to eat a lot of antifreeze, and then it would take them awhile to die," Flood said. "Besides, the odds of all these animals on one block eating antifreeze at once is hard to believe."

Police are asking neighbors if they have used weed killers or mulch on their yards recently. "We don't know why, though, this neighborhood block would be affected and not others," Flood said.

There has been only one clue, he said. A dying dog vomited up the plastic top of a vial that experts are analyzing.

The 5800 block of Bush Hill Drive is quiet but friendly, say residents, most of whom work during the day. At one end is a large, wooded lot. Beyond Larno Lane, at the other end of the block, there have been no reports of animal poisonings.

"It's a shame it had to happen in this neighborhood because it is a nice group of people," Petrulis said. "Now, we have to tell everyone not to bring the children here this Halloween."

To Nancy Hughes, who lost a cat and a dog a year ago, what is most upsetting is the possibility that a neighbor on Bush Hill Drive could be the poisoner. "It's awful to think a neighbor could do this, but who else could it be?" she said.

"It's just spooky," she said.