D. C. fire officials said yesterday that arson was the cause of a fire April 7 that heavily damaged the inside of a building at 405 7th St. NW.

The Development Corporation bought the store-front building and adjoining buildings in February, planing to preserve their facades, which are considered architecturally significant.

Although the fire caused an estimated $100,000 damage to the inside of the building at 405 7th St., the front of the building was not badly damaged.PADC officials said the fire will not affect their plans for the building.

Fire officials said two fires had been set in the building, one on the second floor and one on the fourth floor. The fire on the second floor appeared to have been started with a flammabel liquid, fire officials said.

The fires destroyed the roof of the building and much of the back part of the building , which formerly housed the Marc Anthony clothing store. The building was locked and the locks were still intact when fire fighters arrived, one fire inspector said.

"Somebody took a lot of trouble to get into the building," said Inspector Fred Waren.

The PADC bought the row of vacant buildings on 7th Street in February for $913,000, or almost twice their assessed value. However, PADC officials said they paid less than the value of the buildings as estimated in two recent appraisals.