A French police chief appealed today for information from the public to help determine whether terrorists were behind two attacks against U.S. interests in the southeastern city of Lyons in the past 24 hours.

Anonymous callers claiming to represent mysterious Arab groups have asserted responsibility for the killing of Kenneth Marston, a British-born executive of the U.S. tool-making firm Black & Decker, and a bomb that destroyed the Lyons offices of American Express. But police said that they had no evidence to prove the authenticity of the claims.

After Saturday morning's bombing, police found a slogan, freshly painted in red on a basement wall of the damaged building, proclaiming: "Black et Decker, Control Data, American Express. US go home." The three U.S. companies share space in the building.

The Lyons police chief, Alain Jezequel, said that security in the region was being increased, but he refused to link the two attacks.

A left-wing terrorist group known as Direct Action has been active in the Lyons region. Several leading members of the group recently were arrested in a series of police raids following bomb attacks against military and business targets.

A search of the homes of suspected Direct Action members turned up lists of the heads of several U.S. and British companies in the region around Lyons.

An anonymous caller to the French news agency Agence France-Presse today claimed responsibility for the bombing at the American Express office in the name of the "Arab Revolutionary Front."

Local police said that they could not discount the possibility that the killing of Marston might be linked to the indictments of organized crime figures in Lyons following a major theft at Black & Decker last year. They speculated that the killer might have tried to create a "political smokescreen" by making the murder look like part of a terrorist campaign.