Four people have been arrested in Denver amid fears of a plot to kill Democratic White House hopeful Barack Obama, a local report said on Monday.
Denver-based CBS34 said one of the men arrested had told authorities they were "going to shoot Obama from a high vantage point using a rifle sighted at 750 yards (meters)."
The shooting was supposed to happen on Thursday when Obama will accept the nomination as the Democratic Party's presidential candidate for the November elections at the 75,000-seat Invesco stadium, the television station reported.
One man was arrested on Sunday after police found two high-powered rifles in a rented pick-up truck he was driving, it added.
Another man arrested was reported to be wearing a swastika and was thought to have links to white supremacist groups.
The US Attorney's Office in Denver has scheduled a press conference on Tuesday to discuss the incident, but Attorney Troy Eid told local media he did not believe there was a threat to Obama.
"We're aware of the matter discussed tonight by the Aurora Police Department," Eid said in a statement quoted by the Rocky Mountain News daily.
"Federal law enforcement is working hand-in-glove with the Aurora Police Department. Because this matter is currently under investigation, there is little we can say right now.
"We can say this: We're absolutely confident there is no credible threat to the candidate, the Democratic National Convention, or the people of Colorado."
The alleged plot was being investigated by the Secret Service, which is in charge of coordinating security for the Democratic Party convention as well as the FBI and the joint terrorism task force.