Police discovered a potential plot to assassinate presumptive Democratic presidential nominee Sen. Barack Obama (D-Ill.) after making a traffic stop Sunday and arresting several people on drug and firearm charges.

Police in the suburb of Aurora, Colo., stopped the car in the early morning, according to a spokesman for the Denver Joint Information Center. A local CBS television station first reported the news Monday evening.

The Denver U.S. Attorney’s Office will have a press conference at 4 p.m. Tuesday with more information.

CBS is reporting that at least four people were arrested, and that one said he had come to kill Obama.

Obama received protection from the Secret Service early in his campaign because of the possibility of such threats, and this week’s Democratic National Convention is heavily secured.