As far as mysteries go, this one was a doozy. Stolen car. Illegal drug paraphernalia. A potential bomb nearby. And the fact that the "crime scene" involved a white van and two .223-caliber shell casings made yesterday's mock drama that much eerier.

It wasn't real. Yet on a day when signs pointed to another sniper killing, reality couldn't help but encroach.

Of course, it's not like the region needs to manufacture drama. But as 40 of America's top young science students descended on the District for what promised to be "days of intense, hands-on competition," these middle-schooler's mental muscles were put to some all-too-real-world tests.

And none of those tests was more vivid than yesterday's forensics challenge, at K and Half streets SW.

But first, the students had to get their assignments -- and the police officers helping out had to finish theirs.

So at 8:30 a.m., 2 1/2 hours after authorities began investigating whether the sniper had shot his 13th victim, the students crowded into the Chinese Room, at the ornate Mayflower Hotel on Connecticut Avenue NW. From a Secret Service agent in a black suit, they picked up their orders, and Team 7's sealed envelope said, on the front: U.S. CAPITOL POLICE. Immediately, ninth-grader R.J. Gross held it up to a chandelier and tried to peer through.

"Can't see anything!" groaned the 15-year-old from Lansdale, Pa.

"Still in suspense," answered Nita Bhat, 14, who had come from Miami.

At the same time, Sgt. Michael W. Hays -- the U.S. Capitol Police officer who helped set up the students' mock crime scene -- was finishing up some real police work. Swiftly after yesterday's attack, which came before sunrise, Hays worked a roadblock at the Third Street Tunnel, off Interstate 395 South, as part of a dragnet that webbed across the region.

By 10 a.m., though, both the police and the Team 7 students had assembled by the white van, in back of the Capitol Police's Vehicle Maintenance Division. They were right on the border between neighborhoods of Southwest Washington where marijuana and crack cocaine are often sold.

By 10:15, the students were tearing open their envelope -- "Ah," yelped Nita, "I don't want to rip anything inside" -- then reading their "mission." They would be taught the elements of fingerprinting, drug testing, taking forensic pictures, doing firearm analysis and recovering evidence, after which they would tape off the crime scene and start investigating.

Soon, R.J. Gross was lifting the van's driver's-side floor mat.

"Is there anything there?" Nita asked.

"David! Come here," R.J. yelled. "Dave! Get Dave!"

David Hart, who at 12 was the smallest and youngest of the five teammates, came running with his digital camera and started to snap pictures of the oregano-looking debris under the mat. The students were convinced they had found drugs. Big stuff. And as they continued their investigation, David scooted toward the back seat, where he casually flipped open the cup holder and found -- astonishingly -- a long, gold shell casing.

"Hey," David tried to shout. "Uh."

"It's green grass-like powder," R.J. dictated about the "drugs" to Terrence Bunkley, 14, of Fort Worth. Terrence repeated, as he took notes, "grass-like powder."

"Hey y'all," David tried again in his Lake Charles, La., drawl. "I just found something."

"Do you want to start doing the mirror?" another teammate asked the person with the fingerprinting powder.

Complicating David's position was the fact that he'd been rendered nearly speechless by his discovery. Finally, though, he got Terrence's attention. "Right here." He pointed.

"OHMYGOD!" Terrence shouted. "It's a bullet casing!"

The casing happened to be one of two in the white van. Both were .223s, the caliber used by the sniper -- and the caliber used by the Capitol Police. And there was as little to be drawn from this coincidence as from the coincidence of the white van, which was chosen for the exercise because it had two sliding doors.

Indeed, the ubiquity of both "clues" serves only to show how paltry they are. Instead of being uniquely helpful elements of the real sniper investigation, they are simply, according to police at the mock crime scene, the criminal equivalent of a Starbucks coffee.

For the second year in a row, the Discovery Channel Young Scientist Challenge has continued in spite of terrifying menace, newly unleashed in the Washington region.

Last year, stealth anthrax attacks were killing people, and congressional staff members and postal workers were lining up by the hundreds for Cipro. But still, the students and their families came.

This year, with 13 shot, 10 fatally, apparently at the hands of a serial sniper, fear again could have brought down the Challenge. But still, the students and their family members came.

"My friends said, 'Oh you're gonna get shot -- there's a sniper there,' " said Nita, whose father accompanied her. "And my mom told me to not let my dad do anything stupid, like walk out of the hotel by himself."

Added Terrence, who traveled with his mom: "My friends said, 'Watch out for the sniper.' " But his friends also knew he was getting an all-expenses-paid trip to Washington and a chance at being named "America's Top Young Scientist of the Year," when the winner is announced today.

Plus, like all 40 students, he got an asteroid named after him. Which means that some of his friends, he said, "were pretty jealous."

Capitol Police hazardous materials officers Rob Meikrantz, left, and Jack Dineen, right, brief students, from left, Nita Bhat; Haileigh Stainbrook; Terrence Bunkley, of Forth Worth; David Hart, back to camera; and R.J. Gross. The police set up the students' mock crime scene.Students Stainbrook, Bhat, Bunkley, Hart and Gross are among 40 from across the country participating in a series of science challenges and police work activities sponsored by the Discovery Channel.Nita Bhat of Miami, top center, laughs as students including Haileigh Stainbrook of Piedras, Calif., right, and David Hart of Lake Charles, La., try to take off their hazardous materials suits at the Capitol Police maintenance site in Southwest.Forensics challenge participants David Hart and R.J. Gross of Lansdale, Pa., do preliminary testing on substances found in a backpack left in a trash can.