"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.
Law enforcement sources told CBS station KCNC-TV in Denver that one of the suspects "was directly asked if they had come to Denver to kill Obama. He responded in the affirmative."
One of the suspects told authorities they were "going to shoot Obama from a high vantage point using a ... rifle ... sighted at 750 yards," reported KCNC.
Overnight, U.S. Attorney Troy Eid said in a prepared statement that the case was under investigation, but that he was "absolutely confident there is no credible threat to the candidate, the Democratic National Convention, or the people of Colorado."
Obama will be in Denver 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 - had no immediate comment.
Aurora Police Detective Marcus Dudley said that 28-year-old Tharin Robert Gartrell was arrested early Sunday in a routine traffic stop in the Denver suburb of Aurora. He is being investigated for possible methamphetamine and firearms violations, officials said.
Subsequently, KCNC reports, authorities went to the Cherry Creek Hotel to contact an associate of Gartrell's, 33-year-old Shawn Robert Adolf.
Adolf, who police said was wanted on numerous warrants, jumped out of a sixth floor window of the Denver area hotel. Law enforcement sources said he broke an ankle in the fall and was captured moments later. Those sources say he was wearing a ring with a swastika, and is thought to have ties to white supremacist organizations.
A third man - an associate of Gartrell and Adolf was also arrested. Nathan Johnson, 32, also taken into custody at a hotel in suburban Denver, told authorities that the other two men "planned to kill Barack Obama at his acceptance speech."
Johnson, along with his girlfriend, Natasha Gromek, were also under arrest on drug charges.
Law enforcement in Denver was 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 $50,000 bail on a felony charge of special offender, drug violations. The jail said he did not have a lawyer yet but was due in court Thursday.
U.S. Attorney Troy Eid said in a statement that federal charges were anticipated. Eid did not elaborate, but officials with the FBI; Secret Service; the Bureau of Alcohol, Tobacco, Firearms and Explosives; and Aurora police set a news conference for Tuesday afternoon.