Four months after federal authorities opened a broad conspiracy investigation into the deadly violence at abortion clinics, law enforcement officials said Friday that they have uncovered no solid evidence of a nationwide plot against clinic workers.

Nor was there any evidence, they said, to link Friday's killings in Brookline, Mass., to any conspiracy.President Clinton termed the attack "domestic terrorism," and abortion-rights advocates blamed the government for lax en-force-ment of laws to protect clinics and assailed radical anti-abortion groups for incendiary oratory that they said created an atmosphere encouraging violence.

Abortion-rights supporters renewed charges that the violence was rooted in a small network of people with extreme anti-abortion views who consider killing a justifiable act in defense of the unborn.

"These shooters are supported by a network like any other terrorists," said Ann Lewis, vice president of the Planned Parenthood Federation.

Abortion-rights advocates made similar charges after the shooting of Dr. John Bayard Britton in Pensacola, Fla., on July 29.

In response, Attorney General Janet Reno and FBI Director Louis Freeh ordered a broad investigation focused on determining whether there was a conspiracy involving a handful of radical abortion opponents.

The inquiry has expanded into a large-scale investigation involving a number of federal agencies, including the Treasury Department's Bureau of Alcohol, Tobacco and Firearms.

Reno, meanwhile, dispatched federal marshals to a number of abortion clinics around the country that had been targets of threats or violence. The two clinics involved in Friday's shootings had never received federal protection, apparently because they had never reported serious threats.

Reno, who like Clinton supports abortion rights, condemned the violence at a news conference Friday and said that the federal investigators examining a possible plot would join in the investigation of the Brookline incident.

She avoided the issue of whether federal or local authorities would play a lead role in prosecuting the case, if a suspect is arrested.

Other law enforcement officials said the conspiracy inquiry had never focused on the Boston area, which has not been particularly high on the list of places experiencing abortion clinic violence.

One official said that until Friday the FBI had not assigned agents from its Boston office to examine conspiracy links.

Friday abortion-rights advocates said federal prosecutors should do more to enforce the recently enacted federal law that makes it a crime to interfere "by force or threat of force or by physical obstruction" with anyone who is seeking or performing an abortion or any other reproductive health services.

Federal prosecutors used the law successfully to prosecute an outspoken abortion opponent, Paul Hill, for the murder of Dr. Britton, but abortion rights groups said the Justice Department has been timid in enforcing the law.

"There are death threats but no arrests," said Eleanor Smeal of the Feminist Majority Foundation. "We've got to do better proactively before the shooting occurs."

But under Justice Department guidelines governing domestic terrorism, the FBI's investigation has moved slowly, and people who have followed it closely said the inquiry had turned up plenty of evidence of inflammatory rhetoric shared by the most militant abortion foes, but few signs of any anti-abortion network coordinating or financing attacks.