"It's premature to say that it was a valid threat or that these folks have the ability to carry it out," said a U.S. government official familiar with the investigation. The official spoke on condition of anonymity because the investigation is ongoing.
Obama will be in Denver later this week to accept the Democratic nomination for president.
FBI spokeswoman Kathy Wright confirmed the FBI was investigating the reports but declined to elaborate. The Joint Information Center -- a command set up by Denver, state and federal authorities to field media inquiries during the Democratic convention -- said it had no immediate comment.
Tharin Cartrell, 28, was arrested during a traffic stop early Sunday by police in the eastern Denver suburb of Aurora. In his truck, officers found two rifles, including one with a scope; a bulletproof vest; boxes of ammunition; walkie-talkies; and suspected narcotics, said Aurora police Detective Marcus Dudley.
Police alerted federal authorities, who soon arrested Nathan Johnson, 32, in Aurora, Dudley said. A third man, Shawn Robert Adolf, 33, was arrested at an Aurora hotel, Dudley said.
Dudley didn't say what tied the three men together but that more arrests were possible.
U.S. Attorney Troy Eid said the case was under investigation. "We're absolutely confident there is no credible threat to the candidate, the Democratic National Convention, or the people of Colorado," Eid said in a statement.
Officials with the FBI, Secret Service, Bureau of Alcohol, Tobacco, Firearms and Explosives, and Aurora police set a news conference for Tuesday afternoon.
Law enforcers in Denver are trying to find out whether the reported threats to Obama were valid. "It could also turn out that these were nothing but a bunch of knuckleheads, meth-heads," the U.S. government official said.
Gartrell was being held at the Arapahoe County jail on weapons charges and a felony drug charge. The jail said he didn't have a lawyer yet but one was due in court Thursday.
Officials with the FBI; Secret Service; the Bureau of Alcohol, Tobacco, Firearms and Explosives; and Aurora police set a news conference for Tuesday afternoon.
