Carl Richard Hellman, 16, had been riding his bicycle up Tuckerman Lane in Cabin John one evening last month, headed for the last showing of "The Empire Strikes Back" at a nearby cinema when a green sedan appeared from the darkness, struck him and kept going.

He died on a hospital bed hours later, another victim of a rapidly increasing type of crime in Montgomery County -- the hit-and-run accident. In the first seven months of 1981, there were 711 of the accidents, including four fatalities, compared to only 415 for the same period last year, an increase of 71.3 percent, according to police statistics.

Montgomery police have no explanation for the increase, but they do say that the most serious of these accidents, involving massive vehicular damage or personal injury, are some of their most difficult cases to investigate. Everything from hypnotism and keen-scented canines to tongue-tying crime lab experiments like "pyrolysis gas chromotography" is used. And even then they often can't be solved.

In the 12 jurisdictions covered by the Northern Virginia Planning District Commission, for example, arrests were made only in 1,966 of 7,421 cases in the region in 1980. Police in other jurisdictions in the Washington area don't have such detailed statistics but say the problem is the same.

"You're dealing completely in unknowns," said Montgomery Lt. Jimmy Lee. "Sometimes all you've got is a body, no witnesses and whatever evidence you can glean from the scene . . . . It's like a murder. Sometimes the only person who can tell you what happened is dead."

Solving the cases can be "a long tedious process unless you get lucky," Lee said. Investigators will take the license plate numbers of every car that slows or parks near the scene of a hit-and-run accident. Even the conversation of the person reporting the accident is taped, according to Sgt. Miles Alban.

In the two most recent hit-and-run deaths in the county, arrests were made unusually fast.

In the accident that resulted in Hellman's death, an arrest was made on Aug. 26, 19 days later, when a 23-year-old Potomac man, Shahab Nasrin, turned himself into Montgomery County police. He was charged with negligent driving and failing to stop at the scene of an accident resulting in a death.

When 58-year-old Edward Duvall was struck and killed by a hit-and-run driver as he tried to cross Md. Rte. 124 in Gaithersburg last Wednesday, the man who was eventually arrested and charged in the case -- Allan Holtz of 404 Girard St. in Gaithersburg -- called police the next night, saying he was a witness.

"That's natural. Frequently the perpetrator will drive back by the scene or call in, wanting to know what has happened to the victim," Alban said.

Neighborhoods are canvassed for information and witnesses. Body shops are routinely checked and supplied with descriptions of the type of damage the suspect's car might show. Soil and foliage samples are taken from the scene of the accidents to be compared with particles that might be found on the suspect's vehicle, Alban said.

Authorities will also set up dragnets at the scene at the same time the accident occurred on successive nights. "People are creatures of habit. The majority of people will go back to the same routes they would normally take," Alban said.

Seemingly innocuous shards of glass, plastic and paint scrapings from the victim's car or clothing can be used to determine the make and model of the suspect's vehicle, according to Jack Hughes of Northern Virginia's regional crime lab.

Various chemical tests, such as emission spectrography, which identifies the elements of paint, are then compared to files of the types of paint used on various cars, Hughes said.

"It makes good corroborative evidence," Hughes said. "But paint evidence by itself will never convict a man."

Authorities have used a wide array of methods to gain prosecutions.

Two witnesses were hypnotized in the case of Madeline Danald, a woman who was driving along the Capital Beltway in 1979 when she was struck by a hit-and-run driver and thrown from her car. Under hypnosis, the witnesses supplied the make of the vehicle and a license number, leading to the arrest of two suspects, said Montgomery Lt. Karl Schwarzel.

When the driver of a sports car abandoned his vehicle after striking another car on Montrose Road in southern Rockville a few years ago, it wasn't an elaborate lab experiment, but something much simpler that helped prosecute him.

"The guy had run into some woods nearby and nobody had actually seen him driving the car. So we let a German shepherd get his scent from the car and tracked him down in the woods," said Schwarzel. "It held up in court."