Arson investigators don't need a warrant to seize evidence in a building if they enter while it is afire and remain only a reasonable time, but they do need a warrant to reenter later, the Supreme Court ruled 7 to 1 yesterday.

A person's reasonable expectation of privacy and the protection against unreasonable searches provided by the Fourth Amendment don't shrink "simply because the official conducting the search wears the uniform of a firefighter rather than a policeman," Justice Potter Stewart wrote in the opinion for the court.

The Insurance Information Institute said in a statement that the decision "could limit successful prosecution" of arsonists, whose crimes cause hundreds of millions of dollars in losses annually.

Acting in a case involving a late night fire in an Oakland County, Mich., furniture store, the court held that reentries made to search for evidence within hours after the fire was put out were continuations of the original warrantless entry and consequently were constitutional.

But warrantless reentries and searches made days and weeks later were impermissible and the evidence they produced inadmissible, the court ruled.

In a separate opinion, Justice Byron R. White said the ruling will confuse rather than aid firefighters, "for it will be difficult to predict in advance how a court might view a reentry." Justice Thurgood Marshall joined White.

The dissenter, Justice William H. Rehnquist, wrote that the later searches were "just as reasonable" and consequently just as legal as the initial ones.

The furniture store was in a building leased to Loren Tyler, who ran the business, Tyler's Auction, with Robert Tompkins.

The fire broke out shortly before midnight Jan. 21, 1970. In putting it out, firefighters inside the building found two plastic containers of flammable liquid.

Using portable lights, the fire chief entered the gutted store, examined the containers, concluded there may have been arson and called a detective. After the fire was out, at about 4 a.m. Jan. 22, they took the containers.

In two more searches later that morning, fire and police officials returned and gathered more evidence indicating possible arson. Still more such evidence was gathered by a state police arson expert four, seven and 25 days after the fire. All of the searches were made without warrants and without trying to obtain Tyler's consent.

During the trial, the state buttressed the evidence gathered at the scene with testimony from a former employe that Tyler and Tompkins had told him there would be a fire Jan. 21 and directed him to cover certain flammable objects with mattresses to make them burn better.

A jury convicted Tyler and Tompkins of conspiracy to burn real property, and Tyler of burning property and of doing so with intent to defraud an insurer.

The Michigan Supreme Court reversed, ruling that with few exceptions, officials need a warrant to reenter and search a fire scene once the fire is out and the firefighters have left, "unless there is consent or the premises have been abandoned."

Under the ruling, even the daylight searches the morning the fire was extinguished were illegal. The U.S. Supreme Court, while affirming, held that this went too far.The first morning searches continued the initial entry to fight the fire, while those after Jan. 22 did not, Stewart wrote.

But for the later unannounced entries and searches, the investigators needed only so-called administrative warrants that didn't require them to demonstrate probable cause to suspect illegality, Stewart wrote.

Disagreeing on this point, Justice John Paul Stevens wrote that the Fourth Amendment interests in this case could have been protected either by a warrant issued with probable cause or by fair notice to the property owner.

Justice William J. Brennan Jr. did not participate in the case.