The FBI yesterday joined the search for Ronald Ellis, the suspect in the weekend killings of his wife and five other persons at his suburban Maryland home, amid indications that the Ellises had been experiencing deepening marital problems in recent weeks.

Conflict between Ellis and his wife had erupted publicly two weeks ago when Ingrid Ellis, a D.C. police sergeant, flagged down a police car and accused her husband of stealing her Volkswagen after an argument at a local school, where Sgt. Ellis had gone to pick up her children. Police determined that the incident was a domestic conflict, and no charges were brought against Ron Ellis, according to a school official who described the incident.

Friends and colleagues of both Ellises were aware of their marital problems, and one of Sgt. Ellis' colleagues at the Police Training Academy said Ellis once came to work with a black eye and swollen nose -- injuries he believed she received in a beating by her husband. Still, people who knew Ron Ellis, described by his coworkers at a Silver Spring printing plant as "quiet and dependable," were stunned by the violence that left six dead at the Ellises' blood-spattered home Saturday.

Police yesterday identified the sixth victim in the slaying as Janet Jackson, 31, of the 4000 block of Clark Street in Boulevard Heights, an employe of the Washington Post who was a close friend of Sgt. ellis since childhood. Police had difficulty identifying Jackson because of the severity of her gunshot wounds.

Yesterday, the FBI issued a warrant charging Ellis with unlawful flight to avoid prosecution after Ellis disappeared on Chicago's South Side, where he allegedly had driven with two hostages in a trip that began Saturday night and ended Sunday morning. Ellis had allegedly taken the hostages, a female acquaintance and her 5-year-old son, in Fairfax County and driven with them at gunpoint to a restaurant on a Chicago highway, where the two escaped when Ellis got out of the car, leaving the keys in the ignition.

Ellis then walked into the restaurant, telling a waitress, "I just had a terrible argument with my wife and she drove off and left me," according to Chicago Police Detective Don Long. Ellis asked the waitress where he could get a cab, according to Long, and left the restaurant. In that heavily populated residential and business area off the Chicago Skyway, it would have been easy for Ellis to disppear, Long said. His flight from authorities would have been helped by the $1,200 that the hostage, Mary P. Turner, told police Ellis had in his pocket.

Turner also told police that Ellis said he was traveling to Chicago to seek help from an aunt and uncle who live there. He was carrying a slip of paper with a phone number, presumably the uncle's Turner told police, But the relatives yesterday had heard nothing from Ron Ellis, the detective said.

There was a report yesterday that on Sunday afternoon San Diego police had stopped a man they later though was Ellis, but by last night police officials in the California city said they did not think it was the Maryland suspect. The man who had been stopped was not detained because a computer check indicated he was not wanted on any charges.

The slayings in the Ellises' expensive split-level home at 6700 Coolridge Rd. in Camp Springs followed months of marital discord that seemed to be common knowledge among both Ingrid and Ron Ellis' friends.

"It was obvious that marriage was reaching the rocks," said Lt. Robert White, an instructor at the Police Training Academy, where Ellis had worked for several years before her recent promotion to sergeant. "She was fed up with it," White said of the marriage.

Sgt. Ellis' adoptive mother, Marie S. Withrow of Pleasantville, N.J., said her daughter met Ellis while working as a waitress in a Washington Hot Shoppe, and they were married in 1966 when Ingrid was 17.

Last Wednesday, according to Withrow, Ingrid called home saying that she and Ron were splitting up and that she was fine, according to Withrow. Friday morning, Withrow said she got a telephone call from Ron Ellis, who complained that the breakup was entirely his wife's fault.

Withrow, who knew of their problems, said that Ellis had "threatened" her daughter in the past.

White said that Sgt. Ellis had told friends her husband had beaten her up, and that she sometimes spoke of their problems, but when she came to work one day with a black eye and swollen nose, she said it had happenend when she fell at home.

Sgt. ellis, he said, had been living for the last few weeks with a friend in Virginia, and another colleague said she was looking for an apartment to rent in June for herself and three daughters, Tracy, 15, Tammy, 12, and Monica, 4.

The two youngest girls were killed Saturday by shotgun blasts in a back bedroom of the yellow-shuttered, brick home where they had lived for the last two years. The eldest daughter, Tracy, was staying with friends at the time of the slaying, and is now staying with a Prince George's County policewoman at an undisclosed location, according to police.

Also killed Saturday were two of Ingrid Ellis' friends, Sherry Robinson, 32, of Upper Marlboro, and Jackson, and Jackson's 12-year-old son Tyrone.

Robinson was planning a shopping trip with Jackson Saturday afternoon and apparently had gone to the Ellis home to pick up Ingrid Ellis, who was joining them, according to a friend of Robinson.

Jackson and Sgt. Ellis both had attended the House of Good Sheperd High School in Baltimore. Ellis, who was born in Vienna, Austria, in 1948, became a naturalized citizen in 1955, and was adopted by a family here, according to D.C. police. After high school, she remained close with Jackson and recently talked on the phone with her almost daily, according to Jackson's coworkers in The Post's classified advertising department. Jackson, they said, had talked of setting up a joint household with Ellis.

Instead, the two close friends, Robinson and the three children died in the quiet suburb of Camp Springs, the victims of a shooting that is the worst mass slaying in the Washington area since 1973.

The shootings apparently occurred Saturday after between 3 and 4 p.m., according to police. Ingrid Ellis was later found, dead of handgun wounds, lying in the house near the front door. The other victims had been herded into a back bedroom and killed with multiple shotgun blasts, according to police.

A friend of Ron Ellis from the McArdle Printing Co. in Silver Spring, where Ellis had worked as an apprentice pressman since 1972, said he had visited the Ellis home shortly after 3 p.m., and Tammy Ellis, "who sounded fine," had answered the front door intercom.

The friend, Eddie Rhone, said Ron Ellis came to the front door in his jeans and Nike track shoes and chatted about lending him some picnic tables. "There was no problem at all" said Rhone of their conversation. "Ron was cool. It was pretty ordinary. His eyes were just as clear as they could be." Rhone said that Ellis insisted on bringing the tables over to Rhone's home himself later in the day, and Rhone said he left.

Police said the shootings occurred sometime before 4 p.m., because Ellis allegedly abducted his Fairfax County captives shortly after that.

"It's bizarre," Rhone said, "This is what I just don't understand."

Ron Ellis' father, John D. Ellis Jr., was making funeral arrangements yesterday for his daughter-in-law and granddaughters. "This whole thing has hurt all of us," said Ellis, who came to Washington from his home in Shelby, N.C. "We didn't have any idea that something like this could happen."

Rhone described Ellis as a devoted father who often picked up his two older daughters at school and drove them to their ballet classes before he went to his night-shift job at the plant.

Rhone said he was aware of problems between Ellis and his wife, but did not think they were out of the ordinary.

"Ron was always painting at home, putting down new carpet, putting in an intercom," said Rhone. "This is what kills me. The murders don't make any sense. This is not the Ron Ellis I know."