The interior of a mosque in a town near the German border was destroyed in a fire Saturday, another in a series of suspicious fires in Dutch mosques since the brutal slaying of a Dutch filmmaker by a Moslem extremist in early November. Moslem worshippers arriving at the mosque in Helden, near Venlo, discovered the fire. A spokesman for the Moslem community said arson was suspected. Mosques and Islamic schools have been the targets of fires since filmmaker Theo van Gogh was murdered November 2 in Amsterdam by a young man with Moroccan and Dutch citizenship. A documentary critical of Islam recently had been broadcast on Dutch television when the filmmaker was shot repeatedly and nearly beheaded as he was riding his bicycle along a crowded street. The public nature and brutality of the crime sent shock waves through Dutch society. Although Moslem facilities have borne the brunt of the subsequent fires, Christian churches also have been targeted.