The regional director of a U.S. tool and appliance manufacturer was shot and killed by an unknown gunman today.

An anonymous caller claiming to represent a "small Arab group" later phoned news agencies to assert responsibility for the killing in the eastern city Lyons, but French police treated the claim with caution. They said they were also considering the possibility that British-born Kenneth Marston, managing director for the French subsidiary of Black & Decker Manufacturing Co. of Towson, Md., was killed by petty criminals.

A bomb exploded early Saturday morning in the building that houses the American Express offices in Lyons, and police said a passer-by was slightly hurt by flying debris, Reuter reported. The explosion occurred at 4 a.m. local time (9 p.m. EST), setting the upper floors of the building ablaze and shattering windows in surrounding buildings.

Marston, 43, was shot twice in the chest and stomach on his doorstep in Lyons by a masked man with a hunting rifle, witnesses said.

Police said that there was no way of telling at present whether the anonymous calls were genuine. They noted that Black & Decker France had been in the news recently because of a stock theft that led to indictments against a dozen gangsters in the Lyons region.

But the killing also set off speculation that it was an act of retaliation for the U.S. air raids on Libya.

France tonight announced that it was joining other Western European countries in restricting the movements of Libyan diplomats, reducing their numbers, and delaying visas for Libyan citizens..

Six Libyans have been expelled from France because of alleged involvement in terrorist activities.

The latest measures coincided with a security alert along France's Mediterranean coast following threats by Libyan leader Muammar Qaddafi against southern Europe. Security around U.S. installations has been strengthened and extra protection is being given to 5,000 sailors on shore leave from five U.S. Navy warships visiting the ports of Toulon and Marseilles.

The two calls claiming responsibility for Marston's murder today appeared to be recorded messages. A man speaking French with a strong Arab accent said that he represented a "small Arab group" that would "destroy all American and English imperialist interests in the world wherever they are."