An unemployed 43-year-old Washington man was arrested yesterday and charged with homicide in a case that D.C. police described as a "super mystery" -- the apparently random and unprovoked fatal shooting of a 15-year-old youth who was riding in a pickup truck on Thanksgiving Day 1980.

D.C. police homicide Lt. Carl Alexander called the investigation of the shooting of David Seaton of Seabrook, Md., "an ultimate challenge" in a slaying that he said had "always been a special case to all of us."

Yesterday's arrest followed what police said was an intensive investigative effort that included a search throughout the city and suburbs for automobiles matching the description of the one from which witnesses said the fatal shot was fired at First Street and Michigan Avenue NW.

The suspect was identified as Howard Groves, also known, according to police, as Howard Graves, of 249 Missouri Ave. NW.

Groves also was charged yesterday with assault on a police officer in connection with threats allegedly made against a homicide detective during the course of the investigation of the Seaton homicide, police said.

Police yesterday were reluctant to give details of the investigation that led to Groves' arrest. But one source indicated that police made use of information provided by a witness, and sources also said that a grand jury was convened to hear testimony from one or more persons.

However, police said yesterday they still could offer no motive for the incident, which took place on the gray November day more than 20 months ago as Seaton, a student at DuVal High School, sat in the passenger seat of a pickup truck that was stopped at an intersection for a traffic light.

"Our investigation hasn't revealed any connection" between the victim and the suspect, said Capt. Jimmy Wilson, head of the homicide squad.

Seaton and a friend, who was at the wheel of the 1963 red pickup, were on their way to a construction site to finish some work before returning home for Thanksgiving dinner.

The gunman, according to accounts, pulled alongside the truck and fired a shot from a handgun through the closed window of the passenger side of the vehicle, striking Seaton in the head.

Seaton's friend drove him directly to nearby Children's Hospital where he was pronounced dead at 9:50 a.m.

While investigators characterized the slaying as a random incident so far as could be determined, one source said last night that police are still considering the possibility that it might have followed some brief and sudden confrontation or exchange of words between the victim and the gunman.

Since the shooting, police said, detectives have looked into a series of tips and pored over automobile registrations as part of a step-by-step investigation that one detective likened to completing a jigsaw puzzle.

During the investigation, police said they received a report of another incident in which a shot was fired from one vehicle at another. They declined to say whether any link might have been found between the two incidents.

Little could be learned last night about the suspect, who was said to have worked as a truck driver. A neighbor described him as friendly and "a very nice person."