A massive manhunt expanded throughout the Northeast yesterday in the wake of a bloody Brink's robbery in New York. Federal and local investigators focused on the possibility that the crime was committed by a fusion of a militant white Weather Underground gang and a black urban terrorist group implicated in a string of attacks on police officers.

At the same time, Katherine Boudin, a fugitive Weather leader arrested in the case, was identified by a newspaper reporter as the woman who shared her Manhattan apartment for the past several years. The reporter, Rita Jensen of The Stamford (Conn.) Advocate, said she never knew her roommate's true identity until she saw her photograph after the arrest.

Jensen has hired New York attorney Martin Stolar to represent her. She said that Stolar advised her not to discuss details of her relationship with Boudin, whom she said she knew as Lynn Adams.

According to The Stamford Advocate, Boudin, who had a year-old son, and Jensen lived in a neat, six-story building on Manhattan's upper West Side near the campus of Columbia University for a period of time. Jensen, a 1977 graduate of the Columbia School of Journalism, joined The Advocate in January, 1980.

"I saw the stories and the pictures," The Advocate quoted Jensen as saying yesterday. "She didn't come home last night Tuesday and I knew it was her. That's why I decided to make a statement," she said.

It all began Tuesday when bandits toting machine guns stopped an armored truck in the New York suburb of Nanuet. A guard and two police officers were killed. After Boudin's true identity was learned through her fingerprints, interest in the case quickly increased, as police searched for a possible political motivation behind the robbery.

In New Jersey, where a getaway car used in the bungled holdup Tuesday outside the Nanuet National Bank had led police to an East Orange apartment "bomb factory" Wednesday, state police superintendent Col. Clinton L. Pagano said evidence found there "indicated that the persons who occupied the apartment were involved with the Black Liberation Army and the Weather Underground."

That evidence plus undisclosed "intelligence information" has led investigators to believe that the two groups, both militant offshoots of 1960s protest movements, "have merged," he said.

The connection also seems to be affirmed by an underground film featuring Boudin, released in 1976. At one point, Boudin reads a poem she wrote, dedicated to a black female revolutionary once involved in a shootout with New Jersey police, according to filmmaker Emile DeAntonio. Boudin introduced the poem with the following line:

"I'm a black revolutionary and by definition that makes me part of the Black Liberation Army."

The poem speaks of "daily injustice," and of "anger caged inside, carefully choosing the moment of attack" and ends "I wept for all of us."

Authorities also are investigating similarities between this week's robbery and several others which have taken place in the area in the last few years, according to New York FBI spokesman Joe Valiquette. "We're looking at that hard."

Police already were looking into possible links between a $300,000 Brink's robbery in the Bronx last June, in which a guard was killed, and the shooting of a police officer by suspected Black Liberation Army members in April in Queens.

Evidence found in the New Jersey apartment, according to Pagano, "is not only being analyzed for connection with the Brink's robbery and murders, but also to establish a tie to the escape of Joanne Chesimard."

Chesimard, a Black Liberation Army leader convicted in the 1973 killing of a New Jersey state trooper in a turnpike shootout, escaped in 1979 from the Clinton Reformatory where she was serving a life sentence.

Boudin, in another possible tribute to a Black Liberation Army leader, had named her baby son Chesa, according to her reporter-roommate yesterday.

When the Weather Underground went through one of its periodic splits into factions about two years ago, the dissension centered in part around the importance some members, including Boudin, attached to feminism and also the desire to align with the black liberation movement and Third World causes, according to a source close to the organization.

The May 19th group was "totally Third World-oriented," the source said, "and insisted that there exists an attitude called "white skin privilege." This faction tended to downgrade the accomplishments of whites, including themselves, the source said.

Within the Weather Underground structure, no key members were black because the group members feared they would not be able to spot a black informer or undercover cop, according to the source. The whites tended to come from graduate school, a world the Weather movement knew well, but blacks were more likely to have a different background and the Weathermen felt they could more easily be fooled on this ground.

The trove in the New Jersey apartment included floor plans for six police stations, automatic weapons, a lot of ammunition, documents, bomb making equipment, disguises and false identification materials, Pagano said. Weapons confiscated in the apartment included a 9mm automatic, police sources said, noting that vans and 9mm automatics also were used in Tuesday's robbery and in the Queens police shooting.

Police were still pursuing an unknown number--estimates range from two to eight--of unidentified black men who escaped in the chaotic aftermath of Tuesday's robbery.

Boudin, 38, a fugitive since a Greenwich Village bomb incident in 1970; Judith Clark, another member of the Weather Underground, and two men were arrested as they fled the bungled robbery, in which two police officers and a Brink's guard were killed. The recovered loot totaled $1.6 million.

The four were being held in the Rockland County jail. A hearing on the murder charges is scheduled for Friday.

Federal, state and local authorities from numerous jurisdictions joined in the investigation. Police, following leads, raided homes in New York and New Jersey armed with search warrants. These initially included five search warrants in the metropolitan New York area and one in Mt. Vernon, in Westchester County, according to an FBI official.

The Black Liberation Army, an early offshoot of the Black Panthers, surfaced in the early 1970s and was blamed by authorities for the murders of two New York policemen in 1972.

Also yesterday, a federal Bureau of Alcohol, Tobacco and Firearms official in Albany said he suspected a link between the Weather Underground and the bombing last month of a rugby club in Schenectady where a match with the touring South African Springboks team was scheduled. The bomb was similar to those connected with the Weathermen in the past, he said. And one of the getaway cars Tuesday was traced to a New York woman who earlier had led a violent protest in New York City against the traveling team.