Denver, Colorado/Washington - The arrest of a man with firearms and drugs in Colorado led to two other suspects in a case that authorities believe could have been part of a threat against presidential candidate Barack Obama, according to media reports Tuesday.

Federal law enforcement officials were expected to release more details in the case later Tuesday, the Denver Post reported on its website.

The Democratic Party's presidential nominating convention opened Monday evening in Denver, Colorado. On Thursday, US Senator Barack Obama is scheduled to deliver an acceptance speech at a football stadium to a crowd of 76,000, becoming the first African-American to win a major-party presidential nomination.

Federal law-enforcement authorities gave few details about the possible plot to harm Obama, but a press conference was scheduled for later Tuesday.

"We can say this: We're absolutely confident there is no credible threat to the candidate, the Democratic National Convention, or the people of Colorado," US Attorney Troy Eid told the newspaper.

"The matter continues to be under investigation. We'll provide more information as it becomes available."

The case began when local police stopped a suspected drunk driver in a pickup truck in Aurora, Colorado, outside Denver. The 28-year- old man was found in possession of methamphetamine.

Police also seized two rifles, a high-powered scope-attachment and ammunition from the vehicle.

He led them to his hotel room in Glendale, Colorado, where as police arrived a second man leapt from a sixth-storey window to an awning four floors lower, falling to the ground and suffering a leg injury.

The second suspect and a third man arrested at a Denver hotel were both held on outstanding warrants.

The US Secret Service, which protects the US president, other top government officials and presidential candidates, was investigating, along with other federal police agencies.

"Behavior like this can be characterized as highly suspicious, the ammo, the weapons, clearly would give us great concern," Aurora Police spokesman Marcus Dudley said late Monday. (dpa)