Three men arrested on weapons and drugs charges in the US state of Colorado were not involved in a credible plan to kill Barack Obama, officials have said.

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US Attorney Troy Eid said investigators had found "insufficient evidence of any kind of plot or threat" among the men.

Mr Eid said the threat instead appeared to have been the product of the "racist rantings" of methamphetamine addicts.

Earlier, one of the suspects said his friends may have wanted to shoot Mr Obama at the Democratic convention.

The Illinois senator is set to be nominated as the party's candidate for the US presidency at the gathering in Denver this week.

'True threat'

Speaking at a news conference in the city, Mr Eid said the three "meth heads" arrested on Sunday had not posed a "true threat" to Mr Obama because they simply would not have been capable.

"The alleged threats, hateful and bigoted though they were, involved a group of... methamphetamine abusers, all of whom were impaired at the time," he said.

"The evidence involving the alleged threats does not warrant federal charges now. I must tell you though that the investigation is still ongoing and we are all keeping an open mind," he added.

"From a legal standpoint, the law recognises a difference between what we call a 'true threat' - that's one that can actually be carried out - and the record racist rantings of drug abusers."

The investigation into the alleged plot was triggered after police in the Denver suburb of Aurora stopped a rented truck that was swerving erratically in the early hours of Sunday.

Police said they had found two rifles, two wigs, camouflage clothing, a bullet-proof vest and two walkie-talkies in the truck.

They said they had also found three IDs in other people's names, as well as methamphetamine.

Television interview

The first suspect, 28-year-old Tharin Gartrell, was held on Sunday morning in Aurora after he was stopped by police for driving erratically in the truck.
Mr Obama is due to speak at Denver's Invesco Field stadium on Thursday

Mr Gartrell's alleged associate, Nathan Johnson, 32, was arrested shortly afterwards.

The third man was held after jumping from a sixth-floor hotel window. Reports say Shawn Robert Adolf, 33, broke his ankle and was taken to a hospital for treatment.

In a rambling interview in prison to CBS News, Mr Johnson first denied there had been any plot to kill Mr Obama.

But Mr Johnson, who admits to using the drug methamphetamine, then conceded that Mr Gartrell and Mr Adolf may have had such plans.

"They were here to do that... to assassinate him [Mr Obama]," Mr Johnson told CBS News.

He also said that Shawn Robert Adolf had said in the past that "he didn't believe a black should be a leader of this country".