Kann's department store, a vacant Pennsylvania Avenue landmark, was gutted yesterday by a stubborn five alarm fire that took 12 hours to bring under control and spread acrid smoke over much of downtown Washington.

D.C. fire officials said the blaze, whose cause has not been determined, was the city's largest and most difficult to fight in 30 years.

The fire blazed in large sections of the store until midafternoon, and after that it continued to smolder in small pockets under debris. By nightfall about 20 pieces of fire equipment still surrounded the building. Officials said that some firmen would remain throughout the night as a precaution.

Although the roof collapsed and the inside of the building was virtually destroyed, there appeared to be little dmage to the store's elaborate 19th century facade, which had been covered since 1959 by gray aluminum siding.

Officials said they did not know whether the old facade, which some preservationists admired, could be salvaged when the site is cleared.

At the fire's peak early yesterday morning about 50 pieces of fire equipment and 150 firemen sprayed about 1 million gallons of water an hour in an effort to put out the blaze.

Small groups of firemen made seral forays inside the structure but were forced out by intense heat and smoke, fire officials said. Only one fireman was slighly injured in the fire.

The store occupied three-quarters of a city block bounded by Pennsylvania Avenue and D Street between Seventh and Eighth streets NW, midway between the Capitol and White House. It had been vacant since 1975, when Kann's went out of businees after 82 years.

"It's the hardest fire to fight that we've had in 30 years," said Fire Chief Jefferson Lewis, who arrived at the scene shortly after the fire was reported at 2 a.m. and directed operations for the next 14 hours.

Firefighters were severely hampered by the huge panels of gray aluminum siding that were placed around the store 20 years ago in an effort to modernize its appearance, Lewis said.

In recent months, the store has been the subject months, the store has been the subject of sharp controversy between historic preservation groups and the Pennsylvania Avenue Development Corporation, which planned to tear the building down to make way for 750 units of new housing.

When demolition crews started work in late January, they removed aluminum panels from one 90-foot section, uncovering old columns and curved arches of the building's original late 19th Century facades.

The demolition work was halted when the Interior Department delayed a decision on removing the property from the National Register of Historic Places.

Later, four congressmen asked the General Accounting Office to investigate the historic preservation program of the federally-funded development agency, which is trying to revitalize Pennsylvania Avenue between the Capitol and the White House. The GAO report is scheduled for release in the next few days.

The development corporation paid $4.7 million for Kann's in January 1978. But yesterday Thomas Reagan, director of construction for the agency, said the structure itself "really has no value" because it had deteriorated greatly.

PADC had planned to place parts of the old facades in front of new buildings it hopes to have constructed nearby. But preservationists, led by a group called "Don't Tear It Down," asked that the entire store be saved for reconstructed.

The Kann's store comprised of 15 buildings, constructed between 1985 and 1900, that Kann's put together as it expanded.

Officials said the fire apparently started in a four-story section of the store that runs along Eighth Street, but later spread to a six-story section along Seventh Street.

Battalion Fire Chief Richard Hubscher said that when firemen arrived, the fire was burning on the third and fourth floors. But he said it has not been determined precisely where the blaze began.

Deputy Fire Marshal Joseph Kitt said a team of arson investigators is scheduled to go through the burnedout store this morning.

According to PADC officials, all electricity, ags, and telephone service to the store had long been cut off. But they said there was considerable debris inside and that occasionally vagrants entered the building even though all its doors and windows were sealed.

In asking for permission to demolish it, PADC contended that the store had become a fire hazard.

"Some people said we were just kidding about that," Rita Abraham, public information officer for the development agency, remarked yesterday. "We weren't and now it's happened."

Abraham said PADC had planned to use the Kann's site as a park for several years until it could start construction of new housing, probably in the mid-1980s.

Yesterday Leila Smith, a member of the steering committee of Don't Tear It Down, said the fire in a vacant building was "hardly surprising," but she termed it a "major loss."

"When we looked behind the (aluminum) skin and saw that magnificent facade," Smith said, "we knew there was a kind of architectural detail that never can be replaced."

Early yesterday morning, after the first groups of firmen were driven out of the store, officials called for reinforcements. Within an hour about half of all firemen on duty throughout the city were at the scene. Some off-duty firemen were also called in.

Before sunfise firemen spent two hours removing six aluminum panels on the Seventh Stree side of the store with power saws and air chisels, Hubscher said. At about 11 a.m. a wrecking crane dented one area of siding on the Eighth Street side, and firemen on an aerial platform pulled off four panels.

Heavy streams of water were directed inside the building through both these openings, which fire Chief Lewis said was crucial to containing the blaze by 2:30 p.m. After that, small groups of firemen were able to enter the store with hoses to extinguish pockets of fire that remained. CAPTION: Picture 1, Fire deparment hoses spray water on burning Kann's store in early morning aerial view. By Douglas Chevalier-The Washington Post; Picture 2, Firemen aim water hoses at Kann's fire. Archives Building is in background. Copyright (c) , Linda Wheeler; Picture 3, Smoke nealy obscures early-morning sun as fireman plays hose on Kann's fire. By Douglas chevalier-The Washington Post