They suspect the gunman targeted a schoolboy because officials spent all weekend assuring area parents their kids would be safe at school.
The killer's chilling motive was his own amusement.
"This person is killing for sport," said James Fox, dean of Northeastern University's College of Criminal Justice. "He's turned the Montgomery County area into his own personal shooting gallery."
The boy, who let out a cry and dropped his red knapsack as blood erupted on his shirt, became the marksman's eighth and youngest victim moments after his aunt dropped him off at school.
A single .223-caliber bullet from a long hunting rifle smashed into the boy's abdomen at 8:09 a.m., ripping through his pancreas, spleen and part of his left lung. His aunt bundled him into her car and rushed him to a hospital, where he was on a ventilator last night in serious condition.
Relatives said the eighth-grader normally attends a prayer service before classes at the Benjamin Tasker Middle School in Bowie, Md., but missed it yesterday.
No one saw the shooter or the white box truck he is believed to be driving, possibly with a companion.
Judging from an area of matted grass where a police dog sniffed gunfire residue, the maniacal marksman was lying in wait on a wooded hill above the school, watching through his scope as the kids arrived at the entrance 150 yards away.
A tired Montgomery Police Chief Charles Moose could not contain his emotion when faced with a shot child.
'Getting ... personal'
"All of our victims have been innocent and have been defenseless," he said, a tear running down his left cheek. "But it's stepping over the line - shooting a kid. I guess it's getting to be really, really personal now."
Moose and County Executive Doug Duncan had promised Sunday that the schools would be safe, vowing a massive police presence and noting the killer did not appear to have targeted children.
That might have been a mistake.
"They were trying to reassure the mothers and fathers, but the shooter might have seen that to be a challenge," said former FBI profiler Clint Van Zandt.
"He might have thought: 'I'll take it right to where you said was safe to show you that no one and no location is safe.'"
Cops were posted at schools all over Montgomery County - but the shooting took place next door, in Prince George's County, another Washington suburb.
On a positive note, experts noted that the sniper took a greater risk by leaving his getaway car to climb around in the woods. He also may have left behind useful evidence on his grassy knoll - hairs, fibers or other debris.
"He's taking greater chances," said Van Zandt. "It's part of the usual pattern. You start to feel omnipotent. Eventually, he'll make a mistake, and that's how he'll be caught."
Moose called on Attorney General John Ashcroft to send federal help, and Duncan called on parents to help guard school kids this morning.
President Bush, who has monitored the wave of shootings, decried the "cowardly" shootings and issued a statement sending his "thoughts and prayers to the victims and their families."
In a region already traumatized by crashing planes and anthrax letters, the level of panic in the Washington region reached new heights after the lull of a shooting-free weekend.
Schools across the area locked youngsters indoors, parents rushed to take their kids home and pedestrians everywhere looked nervously over their shoulders.
"It's a fearful time," Duncan said. "As I'm leaving the funeral of one victim this morning, I'm getting briefed on this latest event."
The mad marksman began his reign of terror Thursday, picking off six people at random.
On Friday, the sniper shot a woman in a Virginia mall parking lot about 50 miles south of Washington. She was the first to survive, and her condition was upgraded yesterday to fair from serious.
