The state Attorney Generals antitrust unit is investigating whether Shell Oil Co. is illegally refusing to sell crude oil ... or shutting off pipeline access to Big West refinery on Rosedale Highway. The inquiry was confirmed in a letter dated Friday from Chief Deputy Attorney General James M. Humes to Sen. Barbara Boxer, D-Calif., who earlier this month forwarded to the office allegations that Shell was deliberately manipulating the supply (of oil) in order to force the refinery to shut down.