A federal grand jury indicted a Seafarers International Union employe today on charges of extorting $21,000 from contractors involved in constructing a $15 million Seafarers office building in Camp Springs and enlarging a training center near Piney Point, Md.

Donald William McKay, described by FBI agents as a union building construction superintendent, was charged with demanding cash from three building firms before approving payments to them for work on the two sites.

He also was charged with receiving building materials from contractors for his home in Point No Point in St. Mary's County and then instructing the contractors to bill the union's pension plan and training school for the material.

McKay could not be reached today for comment. His attorney, H. Russell Smouse of Baltimore, declined to comment on the charges.

The 14-count indictment revolves around a Seafarers office building at 5201 Auth Way in Camp Springs and a union training and recreation center near Piney Point in southern St. Mary's County.

FBI agents said the Camp Springs office is being built for $15 million by the Seafarers Maryland Building Corporation, which in turn is owned by the Seafarers Pension Plan. Although not completed, the FBI said, the building is partially occupied by pension, welfare and other international union offices.

The training center is being enlarged with funds from the Seafarers Harry Lundeberg School of Seamanship, according to the FBI.

In the extortion counts of the indictment, McKay is charged with demanding $11,500 from the Heffron plumbing company of Bethesda before approving payments to it by the union for work done. Also, according to court records, he is charged with extorting $7,500 from the Harry Alexander electrical systems company of Bethesda and $2,000 from Hamilton & Spiegel Inc., a sheet-metal fabrication and installation company in Tuxedo, Md.

McKay also was charged with receiving $7,100 in materials for his home from the Alexander company and the J.W. Conway roofing company of Hyattsville, and charging the costs to the Seafarers Maryland Building Corporation and the Harry Lundeberg School of Seamanship.