Oakland Raiders rusher Luke Lawton has been suspended for four games for doping violation, the National Football League (NFL) announced on Thursday. Lawton, a four-year veteran who has once played for the New York Jets and Indianapolis Colts, will miss Oakland's remaining two games of this season and the first two games in next year. Lawton has been banned for taking performance-enhancing substances, the league said, but no specific drug he took was named. According to NFL rules, the league does not announce what substance is involved in a positive doping test. The league policies its own violations but does not follow the World Anti- Doping Agency rules and procedures. The Raiders, who signed Lawton as a free agent before the 2008 season, are 5-9 and conclude the season with games Sunday at Cleveland and Jan. 3 at home against Baltimore. 