Obama will be in Denver later this week to accept the Democratic nomination for president.
FBI spokeswoman Kathy Wright confirmed the FBI is investigating the reports but she declined to elaborate.
Tharin Gartrell, 28, was arrested during a traffic stop early Sunday by police in the 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 Detective Marcus Dudley.
Police alerted federal authorities, who arrested Nathan Johnson, 32, at a hotel in Denver, Dudley said.
A third man, Shawn Robert Adolf, 33, was arrested near a suburban Glendale hotel. 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.
Dudley did not say what tied the three men together but indicated that more arrests are possible.
Officials set a news conference for this afternoon.