An assistant U.S. attorney told a federal magistrate yesterday that the conspiracy with which a Maryland man is charged in connection with eight bombings of abortion clinics and related facilities also includes two firebombings against abortion clinics last year.

Assistant U.S. Attorney Justin Williams made the disclosure during a bond hearing yesterday for Kenneth William Shields, 34, of Laurel, one of three Maryland men charged with conspiracy in the eight bombings.

U.S. Magistrate W. Curtis Sewell set a $25,000 bond for Shields, and imposed a 7 p.m. to 7 a.m. curfew on him. But Shields was not immediately able to post bond and remained in custody last night at the Fairfax County jail.

Phillip C. McGuire, associate director of law enforcement at the federal Bureau of Alcohol, Tobacco and Firearms, said in an interview later that the two firebombings were Feb. 28, 1984, when a salad oil bottle containing kerosene and gasoline was thrown into the Prince George's Reproductive Care Center in College Park, and Jan. 13, 1984, at the Reproductive Care Center in Dover, Del. Each fire caused $100,000 in damages.

Williams did not say why the conspiracy is alleged to include the two firebombings. Shields, Michael Donald Bray, 32, of Bowie and Thomas Eugene Spinks, 37, of Bowie have been charged with conspiracy to bomb the abortion clinics and related facilities, but ATF officials have previously cited only eight incidents.

Government prosecutors had asked Sewell to deny bond for Shields on the basis that he poses a danger to the community. Sewell also turned down a government request that Shields be told not to go near abortion clinics.

Another magistrate, in Baltimore, imposed that condition on Bray, who was freed on bail Wednesday. Spinks remains in jail in Baltimore pending a hearing next week.

Shields' attorney, J. Frederick Sinclair, said his client's family was arranging to post bond and that he expected Shields to be freed today.

In testimony yesterday, ATF Special Agent John J. Mellon said that Shields told ATF agents that he gave Spinks technical advice about chemical formulas and various manuals about building bombs.

"There's nothing to link him to the actual participation, planning, bombing of any of these locations," attorney Sinclair said of Shields. Attempting to focus responsibility for the attacks on Spinks, Sinclair said, "The mad bomber, the man who did all this damage, is in jail."

But Williams noted that, according to ATF agents, Shields admitted to knowing about plans for the bombings and to accompanying Spinks to a Baltimore chemical company to purchase sulphur, an ingredient used in the bombs.