Authorities are trying to determine a motive for the suspected mob hit of the driver for reputed Gambino crime boss John Gotti.

But the Saturday shooting of Bartholomew "Bobby" Borriello, 46, in Brooklyn has set off fears the slaying might be the opening shot in a new mob war.Several theories were being cited for the killing: that Gotti's rivals may be moving in on the jailed Gotti, that a rival family ordered the hit, or that Gotti himself sanctioned the shooting.

"We won't really know for a while, unless someone was `up' on a wire that caught people talking about the killing," the New York Daily News quoted one law enforcement official saying.

Borriello has been described by investigators as a possible hitman in the December 1985 rubout of Gambino crime chief Paul Castellano, New York Newsday reported Monday.