Two other men were arrested in the case.
"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 Gartrell, 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, at a hotel in Denver, Dudley said. A third man, Shawn Robert Adolf, 33, was arrested at a suburban Glendale hotel, Dudley said.
Adolf allegedly tried to escape officers by jumping from a sixth-floor window. He was hospitalized and was being held on $1 million bond for several outstanding warrants involving drug charges. Johnson also was being held on drug charges and his bond was unknown, Dudley said.
It wasn't known if the three men had attorneys, Dudley said.
Dudley didn't say what tied the men together but that more arrests were possible. One of the rifles was stolen, and authorities had traced it to Kansas, Dudley said. He declined to say to whom it belonged.
Aurora police alerted federal officials because of heightened security surrounding the Democratic convention, Dudley said.
"Clearly we found there are federal implications - otherwise we would not have notified them," he said. "The weapons clearly would cause great concern."
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."
