Federal authorities in Denver opened an investigation after police found rifles and ammunition in a rental truck days before Democrat Barack Obama was set to accept his party's presidential nomination.
Three men identified as white supremacists were arrested Sunday after a routine traffic stop that yielded the weapons, a bulletproof vest, walkie-talkies and illegal drugs, officials said. All three had arrest records.
Denver's CBS television affiliate, citing law enforcement sources, reported that one of the suspects indicated that the men had come to Denver to assassinate Obama.
Federal authorities were notified because of the suspicious circumstances and heightened security for the Democratic National Convention, said Det. Marcus Dudley, a spokesman for the Aurora, Colo., police department.
He declined to comment on whether authorities were investigating an assassination plot. He referred that question to federal officials, who have scheduled a news conference for today.