LINCOLN, Neb. (AP) — Nebraska Attorney General Doug Peterson has joined a nationwide coalition warning oil companies about synthetic drug sales at convenience stores.

Peterson signed a letter that was sent to the presidents of major oil companies on Tuesday, expressing concern that employees of their gas stations distribute synthetic marijuana.

The letter from the National Association of Attorneys General says confusion surrounding synthetic drugs allows dealers to convince smoke shops and gas stations to carry the products despite their illegality. The Association called for leaders at BP, Phillips 66, Chevron, Shell, Citgo, Sunoco, Valero, Exxon Mobil and Marathon Petroleum to communicate and enforce standards with store franchisees.

Peterson has been a vocal opponent of marijuana and has thrown his weight behind a legislative bill to strengthen enforcement in Nebraska.