A federal investigation of corruption in the Navy's elite hostage rescue force, Seal Team Six, widened this week with the indictment of a Florida speedboat builder and a subpoena for documents from an Arizona grenade manufacturer.

Sources familiar with the investigation said the probe is now focusing on allegations that some members of the commando team received kickbacks from some of its private contractors. Since the investigation started two years ago, the Navy has imposed stringent new accounting rules on use of classified funds for the counterterrorism team.

A grand jury in Alexandria charged Tuesday that Paul W. Lantz of Hollywood, Fla., owner of a now-defunct boat company, filed more than $300,000 in false claims for labor and material in 1985 for work never performed. Lantz, scheduled to be arraigned Monday, signed a $798,407 contract in March 1985 to design and build several boats for Seal Team Six, according to the indictment.

Joseph J. Aronica, the prosecutor in Alexandria, said the investigation continues into the boat contract and other activities of the unit.

Seals -- for sea, air, land -- are Navy commandos trained for unconventional warfare. Some Seal units are serving with U.S. forces in the Persian Gulf. Seal Team Six, stationed at Dam Neck, Va., near Norfolk, is on call for hostage rescue missions, such as the Achille Lauro cruise ship hijacking in 1985.

Lantz could not be reached for comment.

Earlier this month, Justice Department lawyers in court papers filed in Phoenix alleged "suspected bribery, conflict of interest and standards-of-conduct violations by certain service members of the U.S. Navy in connection" with a $310,000 Pentagon contract with a Phoenix company to build "specialty grenades."

An affidavit by a Navy investigator said there were allegations that another Phoenix company, Aimstar Group Inc., paid three active-duty Navy members "who had roles in procuring, supervising or approving" the contract.

Phoenix newspapers reported that two former enlisted members of Seal Team Six were listed as officers of Aimstar when it was incorporated in 1984. One of them, John B. Mason, pleaded guilty in an unrelated case to filing nearly $13,000 in false travel claims and received a five-year suspended sentence last month. He has been cooperating with investigators and sources said he is a key witness in the continuing investigation.