A federal appeals court ruled yesterday that police seeking criminal suspects must be armed with search warrants -- as well as arrest warrants -- before they may enter the homes of people not suspected of committing a crime.

The ruling by the 4th U.S. Circuit Court of Appeals overturned a lower court decision upholding the actions of Fairfax County police in a 1976 search of two homes there.

Constitutional law experts said the decision appears to fill "a gray area" left unanswered by recent Supreme Court decisions on the rights of police to conduct searches of private homes.

University of Michigan Law Profesor Yale Kumisar, an authority in the field, said the ruling means "an arrest warrant may be enough [for police] to enter the suspect's home, but it is not an 'open sesame' for going into anyone else's home."

Fairfax County prosecutors had argued that an arrest warrant alone was sufficient for a search of any home if the police had "probable cause to believe that the suspect" they were seeking was there. District Court Judge Oren R. Lewis agreed and threw out a lawsuit filed by two Fairfax couples against the county over such searches.

But a three-judge appeals court panel disagreed with Lewis by a two-to-one margin. "An arrest warrant indicates only that there is probable cause to believe the suspect committed a crime; it affords no basis to believe that the suspect is in a stranger's house," the majority said.

Although the appeals court rejected Lewis's reasoning on the searches, it sided with a portion of his ruling holding the officers immune from any monetary damages. "Law enforcement officers should not be held personally liable for monetary damages because they have followed the policy or instructions of their superiors," the appeals panel said.

Neither Fairfax police or prosecutors would comment yesterday on what impact the court's ruling will have.

Deputy prosecutor Steve Merril seemed to hint, however, that the case may be appealed to the Supreme Court. "There has been no Supreme Court case on it," he said, "and there will probably have to be one."

The panel noted that "valid exceptions" to the decision occur when police are in "hot pursuit" of suspects, when there is fear of injury to persons or property, or when they are unable to obtain search warrants from a magistrate immediately.

None of the exceptions, the court ruled, applied in the Fairfax case.

In that case, police appeared at two homes in the Northern Virginia county armed with a bench warrant for the arrest of Susan Wallace Swain, who failed to appear at a child custody hearing. Believing that she was at her parents' home or another house, officers searched both locations and failed to find the woman.

Later her parents, Eldred and Jeannie Wallace and John and Elizabeth DeBiase, who occupied the second house, sued police, claiming that the searches were unconstitutional and seeking monetary damages.

John McNally, a lawyer for the American Civil Liberties Union, which funded the lawsuit, yesterday hailed the appeals court ruling as in "the true spirit" of the Fourth Amendment, which protects individuals from unreasonable searches and seizures.

Elizabeth DeBiase, one of the principals in the suit, said that although she lost the right to collect damages, she was "very pleased by the decision. We joined in it for the principle of the matter, really. That was more important."

The Supreme Court, in its last major ruling on searches, held this spring that police need either an arrest warrant or a search warrant to enter the home of a criminal suspect. The ruling did not address the question of what police need to enter homes of third parties.