Nathan Johnson was arrested at a hotel in Denver. (Aurora Police Department/Associated Press)

After an investigation into what authorities feared might have been a plot to assassinate U.S. presidential hopeful Barack Obama, three men in Denver will be charged only with gun and drug offences.

The three men and a woman, all arrested Sunday as a result of a traffic stop, are not expected to be charged with conspiracy, making threatening statements, or other felonies related to national security.

U.S. attorney Troy Eid described the men Tuesday as "meth heads" incapable of such an attack.

Reports emerged Monday that one of the four people arrested told the U.S. Secret Service the group was "going to shoot Obama from a high vantage point" with a rifle during the Democratic National Convention, which is under way in Denver.

Tharin Gartrell was arrested during a traffic stop early Sunday by police in the eastern Denver suburb of Aurora. (Aurora Police Department/Associated Press)

But Eid, the U.S. attorney for Colorado, said authorities were "absolutely confident there is no credible threat to the candidate, the Democratic National Convention, or the people of Colorado."

Three senior FBI officials said it's unclear whether any of them were serious about carrying out the alleged threats. The officials spoke on condition of anonymity because they were not authorized to discuss the case.

A fourth federal official familiar with the investigation said an assassination attempt was unlikely.

Driving erratically

The three men were arrested separately on Sunday following what police called an "aggressive" inquiry resulting from a traffic stop in which the first of the men was taken into custody.

Police Det. Marcus Dudley said that after pulling over a rented Dodge pickup that was driving erratically at about 1:30 a.m. in the Denver suburb of Aurora, officers found two rifles, one with a scope, in the vehicle, along with walkie-talkies, a bulletproof vest and driver's licences in the names of other people.

An unspecified quantity of methamphetamine was also seized during the stop, Dudley said.

Tharin Gartrell, 28, was arrested at the scene and held at a county jail on $50,000 US bail on drug and weapons charges.

Because of the heightened vigilance surrounding the Democratic convention, police, for as-yet unknown reasons, pursued their investigation to a Denver hotel where, three hours after the first arrest, a second man was taken into custody.

That man, Nathan Johnson, acknowledged to a local TV station that those involved in the case had made racist statements regarding Obama and had discussed killing the Illinois senator on Thursday during his acceptance speech at the Democratic convention.

"He don't belong in political office. Blacks don't belong in political office. He ought to be shot," Johnson told KCNC-TV on Monday in a late-night interview from jail, where he was being held on drug charges.

When asked whether he felt there was a plot to kill Obama, Johnson said: "Looking back at it, I don't want to say yes, but I don't want to say no."

He said he wasn't involved in any plot, and federal law-enforcement officials characterized the plan he outlined as "crude" and implausible, given the distances from which a rifle shot would have to be fired.

About 30 minutes after Johnson's arrest, federal authorities were led to a hotel room in the Denver suburb of Glendale, where a man jumped out of a sixth-floor window to evade capture and suffered a broken ankle, Dudley said.

Shawn Robert Adolf, 33, was sent to hospital and is being held on $1 million US bond for several outstanding warrants involving drug charges.

All three men have criminal records, with convictions ranging from identity theft to assault to drug offences.

The girlfriend of one of the men was subsequently arrested.

The four-day Democratic convention is being overseen by a massive security operation uniting the U.S. Secret Service, the FBI and a joint anti-terrorism task force.

With files from the Associated Press