For months, every person in Washington who owned a black-on-white Cadillac or Buick sedan was a potential suspect in the Thanksgiving Day 1980 slaying of 15-year-old David Seaton, D.C. police officials said yesterday.

Officers hunting for the man who shot Seaton in an apparently random and unprovoked attack at first had only witnesses' description of the suspect's car to go on. In a painstaking operation that was the first step in nearly two years of work, a pair of homicide detectives called up hundreds of city vehicle registration records on a computer and obtained the names and addresses of every owner of a Cadillac or Buick.

Then they began to look.

Nearly a thousand cars later, they believe they found what they were looking for. Yesterday Howard Groves, also known as Howard Graves, a 43-year-old unemployed District man, was arraigned on second-degree murder charges in connection with Seaton's death. He was ordered jailed in lieu of $30,000 bond.

The arrest was a special victory for District police, who had feared at times that they would never track down a suspect in the killing. "The lack of evidence presented so many problems in the case that it appeared that the homicide would never be solved," said Capt. Jimmy Wilson, head of the homicide branch.

Witnesses told the police a white Cadillac or Buick Electra with a black vinyl top and D.C. tags had pulled alongside the pickup truck in which Seaton was sitting, waiting for a traffic light at First Street and Michigan Avenue NW. The witnesses, who police would not identify, said the driver of that car raised a handgun and shot one bullet into the youth's head. Seaton was pronounced dead less than an hour later.

The homicide detectives searching for that car started with a radius of two blocks from the site of the shooting and eventually worked their way outward until they had combed the entire city.

Dozens of times they spotted cars that seemed to fit the description, knocked on the doors of the owners and questioned them until it appeared clear that no connection to the slaying existed.

The detectives also responded to hundreds of "hotline" calls from concerned citizens who had heard broadcast descriptions of the suspect's car and from street cops who had spotted a car meeting the description while on their beats.

"Ninety-nine percent of all the cars we checked had nothing to do with this particular case. You can imagine the man hours it took to run down each car," Wilson said.

Meanwhile, the two detectives gathered evidence that appeared to link the mysterious shooting to other similar ones that had occurred throughout the Washington metropolitan area. Wilson would not go into detail concerning the other shootings, but he said none were fatal and a couple ended with intended victims speeding away to escape fire.

No one has been charged in those shootings. Police said the apparent pattern provided evidence that led to Howard Groves' arrest, but declined to provide any details.

One of the cars examined by the detectives in their lengthy search was a Cadillac belonging to a friend of Groves. Assistant U.S. Attorney William Bowman said in court yesterday that the friend allowed Groves to drive the car on a regular basis.

In December 1980, police first questioned Groves about the shooting, Bowman told the court. At that time, police said, they did not have enough evidence to obtain a warrant for his arrest. Bowman said that after initial questioning by police, Groves left Washington and police were unable to contact him for two months.

In February 1981, Groves was arrested and charged with assault with a deadly weapon in connection with a shooting incident at 14th and E streets NW that police said occurred three days after the Seaton killing. Those charges were later dropped, and police said the case provided no evidence connecting Groves with the Seaton case.

But Thursday evening, detectives arrested Groves and charged him with homicide and with assault on a police officer.

Bowman said in court that two witnesses, including the driver of the truck in which Seaton was riding, identified Groves as the assailant in the Thanksgiving Day shooting.

He also said in court that Groves, a former truck driver, was a violent-tempered man who on two occasions threatened to harm the detectives investigating the case. The charge of assaulting an officer stems from those alleged threats, he said.

Groves' defense attorney told the judge yesterday that his client was at a girlfriend's home in Virginia at the time of the Thanksgiving shooting. He said the allegations against his client "are extremely thin."

A hearing in the case has been set for next week.

The two detectives in the case, Joe Schwartz and Joseph McCann, will be recommended for commendations, Wilson said.